Existing elect' inst' to new CU ?

[tarquinlinbin]

I a CU is exchanged and it is found that some of the existing installation doesnt quite reach where it needs to, what is the correct/acceptable method of extending the existing cable(s)?.

joe

[tarquinlinbin]

PS is it normal practice just to have just the lights on the isolator side of a split CU?. Is there legislation/directive to specify what goes on the CB side and what goes on the isolator side??

ta

joe

[Al Reynolds]

IMHO Crimps are best, done with a proper crimp tool.

Solder is occasionally argued for.

Anything with a screw terminal (choc block / junction box) must be (a) accessible (b) not under strain i.e. hanging

HTH, Al

[Christian McArdle]

Use a correct sized insulated crimp applied with a proper ratchet crimping tool. Cover the lot with heatshrink tubing, preferably remembering to slide it on before crimping the ends!

The legislation is mostly silent except that some bathroom electrics and sockets likely to be used outdoors must have an RCD. If you don't have specific outdoor electrics, then general purpose socket circuits on the ground floor are regarded as being likely to be so used.

However, as a general rule with no legal foundation, I'd divide up the circuits as follows:

Definitely NOT RCD side: Lights Alarms Fridge/Freezer Central Heating* Cooker Fixed heating appliances

• = except when boiler installed in a bathroom

Definitely RCD side: All general purpose socket circuits Electric shower

Personal preference/by convenience: Immersion Heater Dedicated laundry circuit. (i.e. washing machine+tumble+dishwasher) Other fixed appliances

Specical cases: Exterior, garage and shed circuits should be RCDed but should never share an RCD with circuits used within the house. Therefore, they should have a separate RCBO, or come off the non-RCD side and into a dedicated RCD in a separate housing or consumer unit.

Note that in the case of TT earthing, all circuits must be RCD protected. However, this is achieved by using the same method as standard TN earthing, but swapping the main incoming isolator switch with a 100mA time delayed RCD. Alternatively, RCBOs on every single circuit is an expensive, but convenient way to achieve this.

Christian.

[John Rumm]

That begs a question:

I was wondering if there is a preference for location of the RCD on shed circuits? My current arrangement is:

Meter -> Henley -> 30mA RCD -> SWA to outbuildings -> conventional CU in each.

That does have the disadvantage that when you borrow a dodgy electric cement mixer from the neighbours that likes tripping RCDs on damp days, you have to trapse throught the house to turn the RCD back on. So when I replace the CU shortly (with two as below) I could go for:

Meter -> Henley -> SWA to outbuildings -> CU with 30mA RCD in each

This would be more convienient, although that means the SWA itself (or the heavy T&E that feeds it through the house) would no longer have RCD protection.

Or two separate CUs, one with a 30mA RCD, and the other with a normal (i.e. non time delayed) 100mA RCD.

[RichardS]

In this case, however, won't you lose discrimination in the event of a current imbalance in the end circuit of >100mA?

For non-TT, shouldn't the house-side RCD be time-delayed (at 100mA it's not going to provide shock protection, so it's primary purpose must surely be circuit protection?) For TT it's protected by the 100mA time delayed mandatory RCD anyway.

[Christian McArdle]

I prefer the RCD in the outbuilding.

There is no requirement to do so, provided that the cables have been correctly calculated to have sufficiently low earth loop impedence. You already have plenty of cables within the house (i.e. lighting/freezer circuits) and buried under the ground (i.e. your main incoming supply) without RCD protection.

Indeed.

Christian.

[Christian McArdle]

I'm not sure you've understood the proposed solution. You have 2 consumer units, with either a 30mA instant RCD (for protected circuits) or a 100mA instant RCD (for less protected circuits). There is no loss of discrimination. A fault on high risk 30mA circuits will not trip the 100mA RCD, as the current doesn't pass through the 100mA RCD as well. You only need the time delay one if the current goes through both the 30mA and the

100mA one, such as would happen with a split load consumer unit. This is required to prevent the lights going out upon fault in a socket circuit.

Note that for a TT system, the consumer units must be insulated (i.e. plastic) type.

Christian.

[RichardS]

ah, I'm with you now! agreed. ta

[John Rumm]

With my setup, there is no need to discriminate since there are no cascaded RCDs. The outbuildings are fed from a submain split off the incoming supply before the house CU. Currently there is a RCD that protects the outbuilding supply but it is on the source end of the cable. I shall move this to the other end.

An RCDs main purpose is shock protection (since it has no (intentional) over current detection built in). A shock limited to 100mA is still far safer than an unlimited one!

AIUI, you only need a time delay RCD if you are going to cascade other RCDs from it, otherwise in the event of a trip condition (with a trip current above the trip threshold of both RCDs) you would loose discrimination.

For a TT install, it is mandatory that the whole installation is protected by an RCD. The reason a time delayed one is typically used is down to the topology of the wiring... if you have a split load CU and replace the main incomer with a 100mA RCD then you now have the RCD side of the CU powered from another RCD. This is when you use the time delayed RCD as the incomer.

In my case, space restrictions prevent me installing a big enough split load CU, and hence I need to install two. This does however mean I can dispense with the need for the time delay RCD since the CUs are not cascaded.

[Christian McArdle]

Not always. The main purpose of a 100mA RCD on a TT earthing system is to protect the cable from indirect earth faults that might not develop sufficient current to trip an overcurrent device due to poor earth loop impedence. Any direct shock protection is purely incidental.

On a TN earthed system, an earth fault should produce enough current to blow the MCB/fuse, so an additional RCD is not required except where additional shock protection is required (i.e. bathrooms) or where the device may not be considered to be in the same equipotential zone as the supplier's earth (i.e. portable equipment used outdoors, or TT earthing of remote outbuildings). However, most people choose to install RCDs for additional shock protection for socket outlets within the home.

Christian.

[John Rumm]

Fair point when looked at that way... not going to help much with a striaght overload fault, but I guess even a good percentage of these may often be accompanied by a rise in leakage.

My TT system has an earth fault loop impedence of something like 11 ohms. So you could short a final ring circuit directly to ground and still not blow the fuse! (it will be interesting to see if that gets any lower when I bring the earthing up to standard (currently uses the gas pipe and nothing else!)

[Christian McArdle]

But the neutral loop impedence (hence prospective fault current) should already be high enough to trip the breaker. It is only the earth loop impedence that relies on a pathetic earth rod that needs a bit of assistance.

You need to protect against short circuit, earth short and overload. In a TN earthing system, an overload device will usually protect against all three. With TT earthing, it will only protect against short circuit and overload, requiring the additional RCD to protect against earth short.

Obviously using the gas pipe as your earth isn't allowed. However, 11 ohms is well within spec for TT earthing with additional 100mA RCD. A clean 230V short would give you about 21A, easily enough to blow that RCD, but isn't going to be noticed by your 32A breaker, let alone the 100A incoming fuse. An overload or short circuit will however be dealt with by the breaker/fuse.

Christian.

[Owain]

"John Rumm" wrote | Meter -> Henley -> 30mA RCD -> SWA to outbuildings -> conventional CU in | each. | That does have the disadvantage that when you borrow a dodgy electric | cement mixer from the neighbours that likes tripping RCDs on damp days, | you have to trapse throught the house to turn the RCD back on

It also has the disadvantage that you cannot discriminate between lights and tools in the outbuilding; an RCD trip could leave you in sudden darkness with power tools spinning down.

Owain

[John Rumm]

Yup I realise that... (it's on the wrong side of the meter as well IIRC). Having said that it is a metalic gas pipe right back to the street in a heavy clay soil, so I am not expecting a great reduction in impedence even when I install the earth rod and bring all the crossbonding upto scratch.

(must try and not bang the spike through the gas pipe either!)

Even less likely to be noticed by the 30A rewireable fues that is currently there! ;-)

(the main fuse is only 60A)

[Christian McArdle]

Especially as the RCD trip is likely to occur as a result of your power tool being out of control and chopping a cable.

Christian.

[John Rumm]

I always keep my tools under control! ;-)

Moving the RCD to the workshop end would mean I could install a split load CU there and hence keep the lights separate... although to do it right I would be back to having needing either two CUs or a time delay RCD again at the workshop since that is a TT system as well obviously.

It would probably be cheaper and simpler to replace the incomer with a

30mA RCD, which would leave me no worse off than currently, but then add an emergency battery backed light. I was planning to add one of those to the meter cupboard in the house anyway.

[bob]

DO NOT under any circumstances use insulated crimps on solid drawn copper conductors, such as used in Flat Twin & Earth. Crimps are for 7 core+ cable only. There are some manufacturers who claim that you can use their crimps on solid drawn cable, they are misleading you. BS 7671 implies that all parts of an installation should consist of British Standard parts, their is no british standard for "insulated" crimps used in domestic solid copper conductor wiring systems. If it comes to the crunch and this type of connection results in a fire, your insurance company may refuse to pay out. regards Bob

[tony sayer]

In article , bob writes

Well I've just had a rewire done in a house by a NICEC approved electrical firm, thats been around many years and works for local authorities etc, and they have used these for joining 2.5 T&E....

[bob]

Prior to using different values of RCD, (tripI) calculations should be made to ensure that the voltage appearing on exposed metalwork during a fault condition does not exceed 50v. The solution is far more complex than you suggest.

Why is that, perhaps you can list the regulation which applies Bob