CU Replacement on old TT system

[Vessy]

Hi all, first post for me, been a brouser of this site and all it's great info for years.

Looking to replace an old (fuse wire) CU in a 70's house, overhead cables and TT setup. There is a Chilton ELCB installed in the meter tails to the existing CU, the unit is a Type-E 60A 500ma ELCB (Black Unit), no earth connection just the two meter tails in and two out. Will be installing a split load unit comprising of 100A main switch feeding 1 83A 30ma RCB feed and 1 60A 30ma RCB feed, plus relevent MCB's.

Should I leave the old ELCB in place or ditch this as I will have all circuits covered by 30ma RCB's?

The unit works, just doesn't offer me anything above the protection in the CU. Not planning to have any MCB's before the RCB's.

Thanks.

 

[malcolmsanford]

As the ELCB are no longer recognised in the BS 7671-2008 so I would not use it if I were you.

Concerenrd that you said there wasn't an Earth to this as there should be. Have you Earth in the installation??. If there were no earths connected to this then it's basically a double pole switch. It is useful to use as a double pole switch while you replace that CU I know some sparks do that instead of pulling the main fuse.

Are you doing a PIR before the change? If the house still as 70's wiring you might find all sorts of things that might trip RCD's so I would incorporate a full PIR into the change

 

[Vessy]

Standard TT setup, earth from rod to CU then off to circuits. Defo no earth on the ELCB, either it is working as an RDC or not wired correctly!! only 4 terminals so makes sence that it's an RCD. Tried to research the unit but very little info available, especially refering to Type E??

PIR will be done before the change, just getting things ready at the moment, have already identified that we will have issues, borrowed neutrals on up and down lighting circuits (only single reds from lighing fuse into house!), no earth to gas pipe etc etc. and 30 years of DIY additions no doubt!

Still, i agree it makes a good Isolator!

 

[malcolmsanford]

Sounds like it is the current operated type ELCB rather than the Voltage type.

I'm not 100% sure but if it is the current operated type they can be tested with a normal RCD tester I seem to remember this done by someone I know, but sure the lads here will put me straight if I'm wrong.

Well sounds like it's going to be a little diamond of a job mate good luck with that, have you priced for changing that borrowed neutral?

 

[Vessy]

Mates rates!! need I say more!!

Agree with it being the current type of ELCB, not even sure I will worry about testing as circuits will be covered by 30ma RCD's, if anything not requiring a 30ma trip is added then we will have to install a 100A 100ma CU isolator to keep current with 17th. So the old existing ELCB will just be there as an isolator.

Once i get stuck into the PIR I will sort out the borrowed neutral issue, hoping it's borrowed somewhere near the CU, if not then may have to keep both lights and neutral circuit on same half of new CU, OK less than ideal but an esaier option.

 

[Des 56]

Mates rates!! need I say more!!

, if anything not requiring a 30ma trip is added then we will have to install a 100A 100ma CU isolator to keep current with 17th. So the old existing ELCB will just be there as an isolator.

If I have understood, you are inteneding to install an 100 amp 100m/amp Rcd as the main switch ?
If its an all insulated consumer unit,that would not be required
If it was metalclad,then precautions with the tails and the entry would also mean its not required
If it is installed you also to install a time delayed,but you didn't mention it
I would remove the current opereated obsolete circuit breaker as it may destroy any disrimination features of the new board


If I have read the post in correctly Please excuse my "how to suck eggs post "

 

[Vessy]

Presently no, main switch is just a 100A DP isolator in CU

All loads will be through the 30ma RCD's, split inside the CU. As it's a TT system obviously every load has to be through an RCD. If by some chance any additional circuits are required that do not need 30ma protection then i will have to look at removing the 100A isolator in the CU and fitting a 100A 100ma s-type RCD main switch and creating a 3 way split. The house has no immersion heater so the only circuit I can think would be for the boiler if we had an issue with it on a 30ma RCD

I guess all i'm saying is the 500ma function on the old ELCB would never be tripped so it's redundant, 17th states 100ma for unprotected other circuits on a TT system, so if I took a circuit away from the 30ma RCD feed an additional RCD of 100ma would be required for this circuit, also time delayed for discrimination.

Sound good?

 

[brucelee]

hi vessy
what about changing the ELCB for a 100amp DP main isolator switch for future upgrades or just remove the ELCB and use the 100Amp dp main switch in the split ccu, as you are protecting all circuits by
two 30 mA rcd you dont need it or a 100A 100mA time delayed rcd unless as you state if you need a circuit that will not be protected by a 30 mA rcd

 

[Vessy]

hi vessy
what about changing the ELCB for a 100amp DP main isolator switch for future upgrades or just remove the ELCB and use the 100Amp dp main switch in the split ccu

Leaving it in = easy, downside is only 60A rated.
Replace ELCB with 100A 100ms delayed = future proof + quick isolation of CU but most expensive
Remove ELCB = neatest job + 100A 100ms delayed isolator in CU if required

If I were to replace the ELCB with a 100A isolator would I still need the 100A isolator in the CU?

I think we may have to wait until it is all in before deciding, if we need a 100ma circuit then I will have to replace either the old ELCB or the CU isolator with a 100ma device.

As the existing ELCB is 60A I wonder what the main incoming fuse is rated at

Thanks for the advice

 

[brucelee]

hi vessy
you dont have to put a seperate 100A dp main isolator just helps with future isolation and upgrades you can just use the one in the ccu which comes fitted
you only need the 100mA time delayed if you have circuits not protected by the 30mA
the main cut out fuse may also only be a 60 A so will require upgrade if you cant determine the rating enquire with the supplier
you will have to remove the elcb as its only rated at 60A so main fuse will have to be removed whilst carrying out this work by supplier or you but you shouldnt really but it seems everyone does, and then you will be able to tell what type and rating

 

[Will David]

Hello everyone, I've got a CU change on a TT system in the pipeline and have valued reading the information from all of you on this subject. I'm interested to know what sort of test results to expect on a live Ze as all of my previous ones have been on TNS or TNCS systems. Any thoughts?

 

[telectrix]

could be anywhere. if it's above 100ohms, i would look to fitting bigger rod, maybe try a few different locations. everything would have to be RCD. max. Zs 1667ohms.

 

[Will David]

Wow, that was fast!
Thanks very much for your input.

 

[telectrix]

you're welcome. remember to disconnect the main earth from the MET before testing Ze. and 6mm bonding is adequate on a TT system.

 

[Will David]

Hi there, thanks again. I have been looking in the on site guide p 24, Fig 3.2 at TT layouts, will I be able to use a standard split load board or is the use of a 100mA RCD-S type main switch essential? I've looked in various supplier catalogues but can't seem to find one anyway!