Electrical contracting in Glasgow and Renfrewshire - Highlighting the new regulations, installations and current matters to do with all aspects of Electrical contracting - Iain Jamieson Wes electrical
17th Edition: Update for customers in the Glasgow and the surrounding areas
october 21, 2015
Over the next year the electrical industry in the UK, faces Several changes to the electrical regulations. It has already started with the introduction of BS 7671: 2008(2015) - Amendment 3 to the 17th Edition of the IET wiring regulations - on 1st January 2015 and will continue through to 2016.
It is important that electricians and electrical installers and electrical contractors understand what the amendment is and how it will affect their customers and there business
So whats changed?
BEAMA, the independent expert knowledge base and forum for the electrotechnical industry for the UK and across Europe, has said that the primary root-cause of fires in consumer units is loose electrical connections. Additionally, the London Fire Brigade has found that fires involving electrical consumer units have increased to approximately five incidents each week.
It’s clear that a change needs to be made to protect consumers from potential harm.
The London Fire Brigade LFB has been working with Electrical Safety First, BEAMA and other industry organisations to make edits to Amendment 3 requirements that will directly improve personal safety and quell the risk of residential fires.
To address this issue Amendment 3 will provide a degree of enhanced fire risk protection, requiring switchgear assemblies – including consumer units - to have their enclosures made from a suitable non-combustible material, or be installed in a cabinet or enclosure comprised from a suitable non-combustible material, for example steel. This is all covered within Chapter 42 with the addition of Regulation 421.1.201.
MK Electric will be making the mandatory changes to their consumer units, using metal enclosures, which will be available at the end of March. This new range will be available alongside their MK Sentry insulated consumer unit portfolio.
Enhanced fire safety is also referenced in the new Regulation 521.201, which outlines the requirements for wiring systems which are above escape routes, to be supported by fire-resistant fastenings and fixings. All cabling must be supported such that it cannot prematurely collapse when exposed to extreme heat. Once again there is a hint towards the use of metallic materials, although this is not prescribed.
Amendment 3 also puts more responsibility on the installer.
Chapter 41 examines the use of RCD protection on socket outlets. The regulation now requires RCD protection in accordance with regulation 415.1 for socket outlets up to 20A and for mobile equipment with a current rating not exceeding 32A for outdoor use, for all installations. However there is an exception, for socket outlets up to 20A, where the socket outlet is specifically labelled, or where a documented risk assessment determines that RCD protection is not necessary.
Chapter 61 makes a new reference to ‘Skilled person (electrically)’ which has the added condition of the person being competent in inspection, testing and certification work. It also notes that supplies up to 100 amps have a new, more detailed schedule of inspections.
Additionally, for installations greater than 100amps, a model list of items that require inspection during initial verification is provided in Appendix 6.
This list, along with a documented risk assessment of any permitted exceptions to the list must be appended to the Electrical Installation Certificate and the declaration signed.
Dates to you need Know
Over the next year and into 2016 there will be key dates that both manufacturers and installers need to keep in mind to comply with the updated regulation.
• 1st January 2015: BS7671:2008+A3 was published.
Installations designed after this date may comply and be certified to these new standards or be designed and certified to BS7671:2008+A2 (for a maximum transitional period of 6 months)
• 1st July 2015: BS7671:2008+A2 Installations designed after this date must comply fully with BS7671:2008+A3
• 1st January 2016: Regulation 421.1.201 comes into full effect (this doesn’t preclude conformity beforehand)
MK Electric is Ready
MK Electric will shortly be announcing details of its new metal consumer unit range for compliance to regulation 421.1.201. The range will be available from the end of March.
Additionally, MK Electric is offering electric installers and contractors the opportunity to take part in Continuing Professional Development Accreditation for Training to better understand the regulation.
More specifically the session will address how the regulation affects installers, specific changes to be aware of and information they will need to counsel their clientele.
Consequentially, Amendment 3 to BS7671:2008 introduces a new regulation relating to the enhancement of Fire Safety, as follows:
Regulation 421.1.201 – Protection against thermal effects – Consumer Units
Within domestic (household) premises, consumer units and similar switchgear assemblies shall comply with BS EN 61439 3 and shall:
Have their enclosure manufactured from a non-combustible material, or
Be enclosed in a cabinet or enclosure constructed of non-combustible material and complying with Regulation 132.12
Amendment 3 was published on 5th January 2015 and will apply to all Electrical installations designed after 1st July 2015; however Regulation 421.1.201 will not be mandatory until 1st January 2016.
Below is the new amendments for the 17th edition electrical installation regulations Need an electrician in Glasgow? NEW NUMBERING SYSTEM
The Third amendment recognises UK only regulations and these are now
identified by a new numbering system. Regulations with a .100 number
are CENELEC requirements and those with a .200 number are specific
to the UK.
PART 2 - Definitions
Definitions have been expanded and modified, including the introduction
of some new symbols. A list of abbreviations used in the Regulations has
been added. The definition of ‘competent person’ has been removed and
it has been replaced with ‘instructed person (electrically)’ and ‘skilled
person (electrically)’. These changes have been reflected throughout the
wiring regulations.
CHAPTER 41 - Protection against electric shock
There is now a requirement for RCD protection for socket-outlets up to
20A for all installations, however exceptions are permitted for:
a specific labelled or otherwise suitably identified socket-outlet
provided for connection of a particular item of equipment, or
where, other than for an installation in a dwelling, a documented risk
assessment determines that the RCD protection is not necessary
Maximum earth fault loop impedances given in tables 41.2, 41.3, 41.4
and 41.6 have been revised to take account of the Cmin factor. Cmin is the
minimum voltage factor to take account of voltage variations depending
on a number of considerations.
CHAPTER 42 - Protection against thermal effects
Due to the number of consumer unit fires reported within domestic
premises, Regulation 421.1.201 (UK only) has been added and
requires consumer units and similar switchgear to have their enclosure
manufactured from non-combustible material or be enclosed in a
cabinet or enclosure constructed of non-combustible material and
complying with Regulation 132.12.
Please note: This regulation will be implemented on 1st January
2016 to allow for manufacturers to ensure an adequate supply of
suitable cabinets or enclosures is available.
CHAPTER 51 - Common rules
Section 551 concerning compliance with standards requires all
equipment to be suitable for the nominal voltage and also requires
certain information to be noted on the Electrical Installation Certificate.
CHAPTER 52 - Selection and erection of wiring systems
A new regulation 521.200 (UK only) has been included giving
requirements for the methods of support of wiring systems in
escape routes.
The regulations concerning selection and erection of wiring systems
have been re-drafted. Reference to “under the supervision of a skilled
or instructed person” has been removed.
CHAPTER 55 - Other Equipment
A new section 557 Auxiliary Circuits is included. All auxiliary circuits are
defined in Part 2.
SECTION 559 - Luminaires and lighting installation Requirements for outdoor lighting installations and extra low voltage
lighting installations have been moved to new Sections 714 and
715 respectively.
PART 7 - Special Installations or Locations
Section 701 locations containing a bath or shower
There is now a requirement for RCD protection to be provided for:
low voltage circuits serving the location
low voltage circuits passing through zones 1 and 2 not serving
the location.
Appendix 3 - Time/current characteristics of overcurrent protective
devices and RCDs
This includes changes in connection with maximum earth fault loop
impedance to take account of the Cmin factor given in
CLC/TR50480:2011.
APPENDIX 6 - Model forms for certification and reporting The schedule of inspections (for new work only) has been replaced by
examples of items requiring inspection during initial verifications
(which must be appended to the Electrical Installation Certificate).
A small number of changes to the Electrical Installation Condition
Report and associated notes have been made, including a requirement to
carry out an inspection within an accessible roof space where electrical
equipment is present in that roof space.
This summary is not a definitive guide to all the changes introduced by
Amendment 3 and contractors will be required to demonstrate a full and
comprehensive understanding of AMD 3 at their assessment visit. electricians in glasgow westend
As you may be aware there are changes being made to the wiring regulations BS 7671 with Amendment 3 which is due to be published in January 2015. One of these changes is expected to involve consumer units installed in household premises and is focused on the material their enclosures are manufactured from.
What is driving the changes?
Investigation into several fires involving plastic consumer units, by the Strathclyde Fire Brigade, has concluded that a key cause of the fires was substandard cable connections made by the Electrician. These resulted in overheating, which eventually ignited the plastic enclosure.
How has Hager been involved with the proposed changes to consumer units?
As a result of the investigations into consumer unit fires, proposals have been made for changes to the Wiring Regulations. Hager has been closely involved in the development of these changes by providing expert industry liaison with interested bodies including;Strathclyde Fire Brigade, Government and the Joint IET/BSI Technical Committee JPEL/64 which has the responsibility for the content of BS 7671 (17th Edition Wiring Regulations).
Anticipated new regulation
The new Regulation 421.1.201 is expected to state:
Within domestic (household) premises, consumer units and similar switchgear assemblies shall comply with BS EN 61439 3 and shall:
I. Have their enclosures manufactured from non-combustible material, or
II. Be enclosed in a cabinet or enclosure constructed of non-combustible material and complying with Regulation 132.12.
Note 1: Ferrous metal e.g. steel is deemed to be an example of a non-combustible material.
Note 2:* the implementation date for this regulation is the 1st January 2016. This does not preclude compliance with this regulation prior to this date.
What impact will this have?
This would mean that eventually all new consumer units installed in UK homes, i.e. within domestic (household) premises must have their enclosures manufactured from a non-combustible material, or be enclosed in a cabinet or enclosure constructed from a non-combustible material. This is likely to result in an increased use of metal enclosures.
What is meant by ‘within domestic (household) premises’?
It is understood that Regulation 421.1.201 applies to consumer units and similar switchgear assemblies to BS EN 61439-3 inside all domestic (household) premises including their integral/attached garages and outbuildings or those in close proximity.
When will Amendment 3 come into effect?
The third amendment to BS 7671:2008 Requirements for Electrical Installations will be issued in January 2015 and is intended to come into effect on 1st July 2015. Installations designed after 30th June 2015 are to comply with BS 7671:2008 incorporating Amendment 3, 2015.
However, Regulation 421.1.201 does not come into effect until the 1st January 2016. This does not preclude compliance with this regulation prior to this date.
Landlords in England and Scotland will be required by law to install working smoke and carbon monoxide alarms in their properties from October 2015, it has been announced.
According to Housing Minister Brandon Lewis the move will help prevent up to 36 deaths and 1,375 injuries a year after a consultation showed strong support for the measure.
Fire and rescue authorities in Glasgow are expected to support private landlords in their own areas to meet their new responsibilities with the provision of free alarms, with grant funding from the government.
Lewis explained that it is part of wider government moves to ensure there are sufficient measures in place to protect public safety, while at the same time avoiding regulation which would push up rents and restrict the supply of homes, limiting choice for tenants.
‘In 1988 just 8% of homes had a smoke alarm installed but now it’s over 90%. The vast majority of landlords offer a good service and have installed smoke alarms in their homes, but I’m changing the law to ensure every tenant can be given this important protection,’ said Lewis.
‘But with working smoke alarms providing the vital seconds needed to escape a fire, I urge all tenants to make sure they regularly test their alarms to ensure they work when it counts. Testing regularly remains the tenant’s responsibility,’ he added.
According to Communities Minister Stephen Williams it will help to create a bigger, better and safer private rented sector. ‘A key part of that is to ensure the safety of tenants with fire prevention and carbon monoxide warning. People are at least four times more likely to die in a fire in the home if there’s no working smoke alarm,’ he said.
‘That’s why we are proposing changes to the law that would require landlords to install working smoke alarms in their properties so tenants can give their families and those they care about a better chance of escaping a fire,’ he added.
The proposed changes to the law would require landlords to install smoke alarms on every floor of their property, and test them at the start of every tenancy. Landlords would also need to install carbon monoxide alarms in high risk rooms such as those where a solid fuel heating system is installed.
Those who fail to install smoke and carbon monoxide alarms would face sanctions and could face up to a £5,000 civil penalty. This would bring private rented properties into line with existing building regulations that already require newly built homes to have hard wired smoke alarms installed.
New regulations will be laid in Parliament to require landlords to install smoke and carbon monoxide alarms in their properties, and are expected to come into force, subject to Parliamentary approval, on 10 October 2015.
The allocation of funding to fire and rescue authorities to offer free smoke and carbon monoxide alarms to local landlords will be announced shortly.
The British Property Federation said the crackdown was necessary to force the small number of landlords who did not install alarms in their properties to bring them up to standard. It added that although most landlords do everything necessary to ensure the safety and comfort of their tenants, making the installation of carbon monoxide and smoke alarms compulsory will give tenants peace of mind and provide clarity to landlords.
The BPF recommended that the obligation on landlords and their agents should be to ensure that there are working detectors at the time the tenancy starts. There should be an obligation on tenants, however, to maintain and test the alarms and to report any faults.
'It is quick, cheap and easy to install these pieces of equipment, so making them mandatory makes sense, and should weed out those who are currently putting their tenants at risk. While the majority of landlords adhere to best practice and ensure that fire and smoke detectors are installed in every rented property they own, we feel that landlords and tenants can only benefit from this additional safeguard,' said Ian Fletcher, director of policy at the British Property Federation.
Here i have for you a blog about electrical consumer units you know the box that when you have no power it has usually tripped or the fuse wire needs replaced.....enjoy
All cables and connections to the consumer unit must be checked and tested by a qualified electrician. Do not attempt removal or replacement unless you are you are qualified to Part P regulation standards. It is also an offence to interfere with the seals on the electricity meter.
The modern consumer unit is the centre, or heart, of the wiring system in the home. The unit distributes the electricity, via fuses of one kind or another, to the different circuits in the house. The older fuse wires are being replaced gradually by their modern equivalent, the MCB or miniature circuit breaker.
The first one is a single load fuse board where the power coming in is taken through a double pole switch to a live bus bar. Each fuse, or MCB, is clipped onto a DIN bar and the "teeth" of the bus bar are inserted into the MCB's. The cables to the house circuits are connected to the other side of the MCB's. All of this is explained more thoroughly when we deal with the second type of unit, which is the split load unit. The photographs are of a split load fuse board. The ordinary consumer unit is exactly the same in principle without the RCD.
Consumer Unit
Under the 17th edition regulations it is required that every socket is protected by an RCD. This can be done in 3 ways.
A consumer unit can use an RCD as the main switch. This will protect all circuits but if the RCD trips, so do all circuits.
A dual RCD consumer unit can be fitted. This unit has one main switch, two RCDs and each circuit has it's own MCB. This allows the circuit to be divided into two, usually one upstairs, one down. This protects all areas including showers and cookers, but if one trips the other will ensure at least some lights and some sockets still work.
An RCBO consumer unit has one main switch but each circuit is protected by an RCBO which is a combination of an MCB and an RCD. This allows protection in full for all individual circuits and if one trips, all others will still work. As this option is expensive it is only usually used where space is tight for the enlargement of a consumer unit as has to happen when updating to a dual RCD unit.
Just as a matter of interest, DIN stands for Deutsche Industrie Norm and, originating in Germany, is any of a series of technical standards, used Internationally, to designate electrical connections, film speeds and paper sizes. Shown in the consumer unit casing above, it is a metal, pressed bar, to which the MCB's clip. They simply push on via a spring loaded lock at their back.
Split Load Board
A split load board is designed for total safety and incorporates an RCD (Residual Current Device, shown as E in the picture above) as well as the double pole switch (D).The split board shown here has one RCD and is a simple version to show the user how the board works.
An RCD is a manually operated isolator switch, but it is also an automatic safety device that will trip and cut off as soon as it senses an earth fault. There are a number of different ratings for current and sensitivities to current leakage available, so you will need to understand which are the appropriate ratings for your home. RCD's also work by detecting an imbalance between the Live and the Neutral conductors and this feature is a requirement for all Class 2 double insulated equipment.
As they are very sensitive, it is not practical to fit one RCD to protect the whole house. If a fault develops on one circuit, all circuits would be switched off immediately...This would, most often, leave you in the unnecessary position of having no lights or power. Because of this it actually contravenes the most recent (16th Edition) wiring regulations. This is easily prevented by using an RCD in conjunction with a main, double pole isolating switch so that it protects only some of the more vulnerable circuits. This then becomes a split load consumer unit and is shown above. The key to this photo is..
A: Neutral terminal block for the main isolating switch side of the unit
B: Neutral terminal block for RCD side of the unit
C: Earth terminal block (takes both sides of unit)
D: Main double pole isolating switch
E: Generally 80 - 100 Amp RCD with 30mA (milliamp) sensitivity
F: Neutral link cable from Main switch to terminal block
G: Live feed from main switch to RCD
H: Neutral link cable from main terminal block to RCD
I: Neutral link cable from RCD to RCD neutral terminal block
RCD's
The use of RCDs is necessary, on two occasions, to meet the wiring regulations. The first of these is to protect any socket which may be reasonably expected to supply equipment outside the house (Strimmers, lawn mowers, hedge trimmers, pond pumps, sheds, lights etc etc). This rule pretty much covers every single socket in the house so the ring main should be placed on the RCD protected side of the consumer unit.
RCDs have a test button which creates an earth leak with a resistor, and it is usually recommended that they are tested once a month. An information label near the consumer unit explains this The resistor passes more current than that required for tripping, and the duration of the test is not limited, so testing in this way does not provide proof that the unit is working to specification. RCDs can also be switched off manually and can take the place of the isolator switch in the consumer unit, if they break live and neutral.
The second condition applies to any circuits where an earth fault current is not sufficient to blow the fuse, or trip the switch, in the designated time allowance made in the regulations. This generally applies to higher Amp rated appliances (showers, cookers etc) but it should always be checked with an electrician as to which circuits need to be placed on the RCD side of the consumer unit.
MCB's
The Current ratings which MCBs are designated to deal with are now conforming to the international equivalent of our old imperial ratings. This standard is called the Renard current rating and supercedes the imperial ratings as far as MCBs are concerned. A 5 amp fuse can be replaced with a 6 amp MCB. A 15 amp fuse becomes a 16 amp MCB, a 20 amp fuse can be a 20 amp MCB, 30 amp fuse = 32 amp MCB and a 45 amp fuse must now be a 40 amp MCB.
Consumer Unit with MCB and main double pole isolation switch
Consumer Unit with MCB's and Bus Bar
Top left shows the mains double pole isolating switch and an MCB clipped into place on the DIN bar. Top right shows two MCBs and the bus bar. When the MCBs are clipped onto the DIN bar, the bus bar is inserted into the live terminal at the bottom of the MCB. When screwed tight this gives live feed to all the MCBs.
Main double pole isolation switch on din bar
live and neutral coming into the main double pole switch from the meter and earth terminal block
Top left shows the live (K) and neutral (J) coming into the main double pole switch from the meter. These cables are called tails, and in this instance, with this type of unit, are 16mm cables. This will vary and you should always check with a qualified electrician. Top left shows the earth cable from the meter (L). This screws into the earth terminal block.
Live connected to MCB with neutral going to RCD neutral terminal block
Top right of the above image shows how a circuit cable is introduced to the unit. In this case it is a 2.5mm cable for the ring main. The live (O) is fixed into the top of an MCB on the RCD side of the unit, while the neutral (M) goes to the RCD neutral terminal block. The earth (N*) (which has been left unsheathed for clarity ). Each circuit cable is fed into the consumer unit and connected to an MCB of the appropriate rating.
Note that this represent major electrical work, and should not be attempted unless you are confident that you understand the technicalities involved, and can produce an adequate standard of workmanship.
You will also need access to specialist test equipment such as an earth loop impedance tester, and insulation resistance tester, a low ohms meter, and a RCD tester (or equivalent multifunction tester).
You should have access to a copy of the IEE on-site guide and a copy of the IEE Wiring regulations.
Note that most of the work described here would be classed as a "notifiable work" under Part P of the building regulations.
Additional Safety Advice
When carrying out earth fault loop tests, and insulation resistance tests on virgin installations, one should take great care to ensure that no other occupants of the house are put at risk of exposure to high touch voltages should it turn out there is a failure in some part of the protective infrastructure, and that adequate warnings are given before testing.
Reasons for a change
There is often an implicit assumption that a modern CU with resettable Miniature CIrcuit Breakers (MCBs) will be "better" than an existing one that has cartridge or re-wireable fuses. It is important to understand that both types of fused circuit protection are still permissible in the current wiring regulations, and can offer the required levels of protection. There are also disadvantages to changing from fuses to MCBs in some cases.
Reasons to change
You need provision for more circuits
The existing CU is damaged in some way
You need to better integrate (or provide for the first time) RCD protection for circuits.
You have a rewireable fuse CU and there is a risk that uninformed people may attempt to re-wire a fuse with the incorrect rating wire.
You have older PVC T&E power cabling with undersized earth wires and re-wireable fuses.
You need to separate out circuits to allow independent control - say for time switched electric heating, or for a power feed to an outbuilding.
To rationalize or simplify an existing system which is complex (either just to reclaim space, or to make use of the system less prone to errors).
Potential pitfalls
In many cases a new CU fitted with MCBs will be upgrading an older CU fitted with fuses (either cartridge or re-wireable). There are a number of problems that may manifest as a result:
Nuisance trips. Compared to fuses, modern MCBs react more quickly to very short term overloads, and may result in loss of power to a whole lighting circuit when a bulb blows.
Discrimination: it can be harder to ensure that the circuit protective device nearest to a fault will be the only one to open when you have cascaded MCBs - sometimes upstream fuses interoperate better with downstream MCBs
Expense: Changing a CU can be expensive, and may not bring significant benefits in overall safety. There may be other more serious problems with an electrical installation that are better addressed first.
Extra work: Fitting a CU with RCD can often result in the installation not working initially due to hitherto unnoticed faults in circuits such as a borrowed neutral or higher than expected earth leakage. While discovering these faults is not a bad thing, it can force the investigation and repair of a number of other issues not directly related to the original task planned, causing unexpected cost and delay.
Note that steps can be taken to minimise these potential problems, such as using HRC fuses where more appropriate (e.g. feeding submains to outbuilding CUs), or using type C MCBs on lighting circuits etc.
Planning
Location
Consumer units typically need to be within 2m of the electricity meter unless an additional switch fuse is fitted to protect the tails. They should not be mounted on the electricity suppliers meter board (although this is quite often seen). An ideal location should also make access to the CU easy without need to resort to ladders or climbing past obstructions - since you may be doing it in the dark.
Ways
The spaces into which circuit breakers and other devices may be fitted are called "ways". Generally each device (and hence often each circuit) will take at least one way. When selecting a new CU, it is wise to choose one with enough ways to accommodate all of the proposed circuits, and to leave a few spare ones for future applications. Note also that some devices that can be mounted in a CU such as contactors, time clocks, bell transformers etc, may be wider than a standard single module and hence take more than one "way", and some of these more exotic devices may also require a space to be left beside them to facilitate cooling.
Any modern CU will typically feature at least one RCD, and quite probably more in new designs.
Split Load Vs Multiple
When selecting a CU, one option that is often overlooked is to use more than one CU rather than one big split load one. For example, it may be simpler to position a pair of smaller CUs in a confined space giving adequate free ways.
In the case of houses with TT Earthing, the normal 16th edition style single CU solution involves a 100mA time delayed RCD in the position of the main switch, with the 30mA RCD in the split load position (the time delay resolving the problem of discrimination between the RCDs under fault conditions). Using two CUs will allow a normal 100mA RCD to be used on one CU dedicated to circuits that do not require direct contact protection, and the other for socket circuits etc. This will save the cost of a time delayed RCD since there is no need to cascade RCDs in this case.
Service Connector Block / Henley block
Multiple CUs can each be fed from their own set of tails. A Service Connector Block (aka "Henley Block") is a large junction box that can be used to split the tails from the meter or main switch enclosure, to allow the feeding of multiple CUs. Note these are available in single pole and double pole. The double pole devices will accommodate splits of both the Line and Neutral tails of a typical domestic supply in a single unit.
When more that one CU is fitted, it is often worthwhile fitting an additional single "main switch" so that the capability to kill all power to all circuits with one action is retained.
Selecting Equipment
The immediate choice you will be presented with is the choice of metal clad or "insulated" (i.e. plastic) CUs. On TN [1] systems either can be used. The metal clad ones are better if you need to terminate cables like SWA with glands. The plastic ones are better for smaller installs where the ability to easily cut out only small sections for cable entry is handy.
With a TT install you will need to use a plastic CU since this minimises the possibility of a phase to earth fault occurring inside the CU before the RCD (and hence never being cleared due to the high earth fault loop impedance of the local earth spike).
One should also check that the breaking capacity of the selected protective devices is adequate. Typically modern MCBs can break fault currents up to 6kA. This is usually more than adequate. However if you live in a large densely populated city, or very close to a substation then you should use the appropriate test equipment (most earth fault loop testers will have a capability to measure the prospective short circuit current) to measure the maximum prospective short circuit current to ensure it is below the maximum limit that can be safely broken by the selected MCBs. In situations where 6kA is inadequate, the use of HRC cartridge fuses may be more appropriate, or using some of the ranges of consumer unit and circuit breakers with higher breaking capacity designed for industrial or commercial use.
Finally, many CUs will have their main isolator switch on the right hand side, however this is not universal and different brands of CU may favour placing the main switch on the left. Some can be reconfigured to your preference. The position of your electricity meter and the length of the tails may dictate that tails entry on a particular side is required.
Other work
One job that frequently needs doing with a CU replacement is installation or upgrading of the main equipotential bonds between the main earthing point and the metallic services entering the building.
Starting work
Plan your work schedule carefully. Make sure you have enough time allotted to complete (or at least advance it to a appropriate intermediate stage) in the time / light available. Make sure any cordless tools you need are charged, and that batteries in test equipment are ok. You will also need to ensure you have enough natural light or suitable additional lighting to carry out the work safely. Head mounted LED torches are ideal for this sort of work. Again make sure you have spare batteries. Ensure you have adequate spare cable / wire to hand plus crimps and junctions boxes etc should you need to extend circuit wires.
Disconnecting the power
Prior to replacement of the CU it is necessary to isolate the power feed to it. Some installations have an isolation switch for this purpose, however many don't and the other most readily available method is to remove the main service fuse. The official way to do this is to seek a temporary disconnection of the supply. More details on the procedure can be found here.
Removal of the electricity distributor's main fuse yourself is not legal, however it is common practice. Unofficially it is reported that many of the supply companies would prefer electricians do this, rather than take the risk of working live.
Great care must be taken when removing the fuse. It must only ever be pulled when there is no load on any of the circuits (i.e. all existing CU(s) turned off). These fuseholders may be of historic design (possibly dating from the 1940's or earlier), and may lack even fairly basic safety guarding. Touchable metalwork inside the fuse enclosure can be assumed to be live. Please see the additional notes in this article.
It is possible to remove the tails and tape the ends over, but this is not recommended, as it does constitute a risk of injury, loss of sight and to life. Live working should not be attempted unless one has the correct equipment and knowledge to allow this to be done safely.
Labelling
Start by identifying and labelling all the existing cables that enter the current CU before disconnecting the wires. One mistake here can cause a fair bit of trouble later!
Removing the Old CU
Depending on the design of the old CU it may be necessary to remove the fuses or circuit breakers first to be able to disconnect the circuit wires. You will probably need to straighten the wires to allow them to be pulled through the cable entry holes in the old CU.
Take care if there is any old rubber cable present, since these will have fragile insulation easily damaged. As a general rule rubber wired circuits are not fit for re-use typically due to insulation breakdown, and undersized protective conductors. In some limited cases it may be possible to re-connect a rubber insulated cable, in which case great care will need to be taken to not damage the insulation while disconnecting and removing the wires.
Intermediate tests
Once the old CU is out of the way, this is an ideal time to make checks on the existing circuit wiring. Round trip low ohms tests on ring circuits can now be carried out easily since both ends of the ring are readily available. Insulation resistance tests on individual circuits are also easy to carry out. If you find faults at this stage (like a ring circuit that has a broken loop) you may need to mark this for later attention rather than divert activity away from the main task now. You can always leave one circuit disconnected when reconnecting at the CU later. Some simple tests at this stage can also help identify likely causes of nuisance RCD trip later (like borrowed neutrals or earth / neutral shorts).
(recording the test results as you go can save some work later)
Installing new CU
Prepare the new CU
First check and setup the CU as you require it. This may mean you need to choose where splits will occur on split load units. Often this will require the cutting of the live busbar to the required number of ways for each section. Even if the CU is apparently already setup correctly you need to make sure that all connections are tightly made and in the right place.
The New CU does not have to be in the same location as the previous one. Obviously if it is some distance away then the existing circuit wires will need to be extended. Even with a CU in the same location as before it is not uncommon for wire extensions to be required inside the box simply due to different layouts. Inside the CU individual wire extensions may be crimped on. Outside the CU then either junction boxes will be required, or fully insulated cable crimped joints be made.
Fix the Equipment
You may find it easier to fit any service connector blocks and smaller switch enclosures before the main CU is in place.
Fix the CU in place - if this is going to require masonry fixings take care that you are not going to drill into any buried wires!
Connect the supply
Next the main earth connection plus the supply tails from the meter / switch / service block can be installed. These will need to be done using suitable size tails (typically 16mm² for supplies up to 80A, and 25mm² for 100A supplies). Once the main supply connections are in place you can carry out a visual inspection and a a couple of quick sanity checks with a multimeter to ensure you have got the polarity correct and there are no shorts on the supply.
Install protective devices
Depending on the design of the CU it may be easier to install all the protective devices at the start, or it may be easier to do them one at a time as you reconnect each circuit. The latter approach is least open to error.
Re-connect circuit wires
Connect each circuit in turn following your labelling. If you have new circuits to introduce that were not connected to the old CU it can be worth leaving those until you have restored the original functionality and tested it.
Labelling
The MCBs or fuses should be labelled to show what circuits they supply. Installations using both wiring colour codes are required to have a notice fixed by the CU stating that both colour codes are in use. It is a good idea to label as you go to save confusion later.
Testing CU main wiring
Once all the circuits are connected, another visual inspection can be done, and you can go over all the screw connections again to ensure they are tight. Check busbar connections to the main switch / RCD as well. A final sanity check with a multimeter will also catch any silly mistakes.
You can carry out a full installation insulation resistance check at this point also (taking care to protect any sensitive electronic equipment wired to the circuits first!)
Reconnecting power
With all the CUs turned off the power to the CU can now be restored (fuse replaced etc). All the individual ways should be turned off on the CU before turning on its main switch.
At this stage you can carry out full RCD tests using an appropriate tester.
Each of the individual circuits can then be turned back on one at a time, and functional checks be carried out.
Resolving Problems
If all goes well you should now have a working electrical system again. However in a minority of cases you could meet problems. These may be ones that you have introduced during the CU change, or often, they are ones that were always there but did not make their presence felt until now.
Nuisance RCD trips
It is not uncommon for a new CU to trip the RCD as soon as power is turned on. There are a number of common causes of this, which are addressed in the RCD article.
MCB trips
MCBs trip when too much current flows in the circuit they supply. Since the odds of an appliance fault developing while you replace a CU are small, the likely causes of MCB tripping when a CU is replaced are:
a circuit miswired during the works (eg when fixing another fault)
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