100ma front end rcd and metal clad ccu

[soulman]

Hi,

Sorted out a fault today on a tt system earth fault on the rfc. The consumer unit was metal clad & the front end rcd was 100ma. The tails had been split via a 100a main switch within an enclosure to a second consumer this was plastic & contained a 30ma rcd feeding an outdoor supply to the shed. I have told the customer that the metal clad ccu unit should be changed and also all circuits should be protected by 30ma rcd as it is a tt system.

My question is where they have split the tails with a 100a main switch could i change this with a 30ma rcd and leave the metal clad in. The customer is 92 years old. To me it seems the simplest solution but i'm not sure.

Question 2. What coding would you give on EICR for metal clad ccu, and for having a 100ma rcd rather than 30ma on a tt as i would have thought C1 or C2 but the electrician i was with said C3

Cheers

 

[Andy™]

there is no reg to say TT circuits must have a 30mA RCD.

the board should have an RCD before it though. you coudl install a 100mA TD RCD before it.

and maybe you should stop doing EICR until you know the regulations.

 

[steptoe]

are you really a spark?

or a 5WFW?

Im not sorry if you find that offensive, but your question tells me you are a a 5WFW so please be honest with your answer,

ADMIN

we really need this box quickly

 

[samson337]

Be kind and let the old dear live to a hundred, coz after reading that she might not if you do any work on the property,it seems that your well out of your league, and don't mean to affend

 

[soulman]

yes i am fully qualified but still learning doing little jobs i thought all tt systems should have 30ma protection and metal clad ccu should not be used. And i'm not doing an EICR i'm just asking a question from the more experienced. as unfortunately one spark tells me something different from another spark.

---------- Post Auto-Merged at 22:22 ---------- Previous post was made at 22:21 ----------

Th e customer is 92 & unwell i was just looking for a simple solution rather than changing the ccu.

 

[SLIPSHOD & SLAPDASH]

I cant remember the exact wording but a insulated gland that clamps the cable is acceptable if using a metal enclosure.

 

[Andy™]

yes i am fully qualified but still learning doing little jobs i thought all tt systems should have 30ma protection and metal clad ccu should not be used. And i'm not doing an EICR i'm just asking a question from the more experienced. as unfortunately one spark tells me something different from another spark..

do they not teach TT systems & RCD requirements to become qualified these days?

 

[NozSpark]

I've probably said or asked this before,,, but IIRC so long as the cables are bushed into a metal clad CU correctly then you don't need "up front" RCD protection for the tails???.?..

and as the others have said; TT final circuits, whilst they do need RCD protection, do not necessarily require 30mA protection

 

[soulman]

I was taught if it was metal clad on a tt system it either had to be changed to a plastic enclosure or a 30ma rcd installed before it. So i thought by replacing the 100a main switch which splits the tails to a 30ma rcd unit i would be achieving this and also protecting circuits as a temporary measure. If this is not acceptable please point me in the right direction.

 

[NozSpark]

I cant remember the exact wording but a insulated gland that clamps the cable is acceptable if using a metal enclosure.

Beat me to it

 

[Andy™]

I was taught if it was metal clad on a tt system it either had to be changed to a plastic enclosure or a 30ma rcd installed before it. So i thought by replacing the 100a main switch which splits the tails to a 30ma rcd unit i would be achieving this and also protecting circuits as a temporary measure. If this is not acceptable please point me in the right direction.

an RCD on TT is for fire protection. this is why 300 & 500mA RCD's are made.

 

[soulman]

Andy i was taught at college that all circuits should be protected by 30ma rcd on a tt system.

 

[Andy™]

Andy i was taught at college that all circuits should be protected by 30ma rcd on a tt system.

then you were taught wrong.

it is possible (although unlikely & rarely done) to have a TT system without any RCD's

 

[soulman]

I also realise that 100ma and above are for fire protection but what about additional protection needed on a tt system as efli path too high. if i'm barking up the wrong tree put me right. tell where i'm going wrong.

 

[samson337]

Stuffing gland

 

[SLIPSHOD & SLAPDASH]

MK used to sell a kit exactly for this purpose, dont know if its still available.

Last year i had a job to do where the incoming 50mm SWA came from a TNCS supply 75m away & it had to be terminated into a metal feeder pillar (cabinet) I adapted a blue water pipe fitting & it worked perfectly like a huge stuffing gland to hold the cable into the enclosure. I double sleeved all 4 cores and made an additional clamp from a piece of plastic trunking to hold the cores rigid as they terminated into the main switch. It was one of the jobs shown on our annual inspections and passed with flying colours.

Edit, should add that this was made into a TT system & we had to sink 5 rods to get a decent reading

 

[NozSpark]

Typically in a domestic setting all circuits (newly installed) will require 30mA RCD protection no matter what the installation type is due to burried cables (<50mm) and socket circuits.

However TT installations need RCD protection for other reasons,, protection of livestock and against fire which could be 100, 300 or 500mA. This would suffice for surface clipped cables etc that don't fall under the <50mm, bathroom ccts or socket outlet regs

 

[OnOff]

[quote name='Andy

 

[SLIPSHOD & SLAPDASH]

MK 5802 http://help.tradingdepot.co.uk/electrical/mk/sentry-prod.pdf Page 17. That was hard to find, kept getting 'remote control tent light' ???

5802s

RCD INSULATION KIT

For use when an RCD is

installed in a metal enclosure

in an area of high earth loop

impedance.

Kit consists of male and

female screwed ring for fitting

into a 32mm knockout and

clamp to hold cables secure

 

[soulman]

We were taught very little about tt apart from eli being too high therefore rcds need to be used for fire protection & additional protection, we were also taught never to use metal clad ccu. we were also taught what andy has said that tt do not need rcd according to the regs but they could not really give an example of when you could do this. Electricians on this forum i have a great respect for and would like my original post to be answered constructively. if you want to pull me to bits feel free! but tell me where i'm going wrong please.