TNC-S system with a TT sub-board

[Thomas02]

My neighbour had problems with her RCD tripping in the main consumer unit and she indicated it may be linked to her fish pond set-up outside the property which is fed from a plug socket in the garage.

We investigated that this particular garage circuit was on a B16. We powered up the breaker but there was no power from the garage. Further investigation led us to find an RCD protected sub-board in the garage.

The feed for the RCD protected sub-board is also RCD protected - is this correct or would it be more practical to move the feed B16 breaker over to the non-RCD side?

The feed exits the house via an IP rated enclosure and two-core SWA is used for the feed to the sub-board. The main earth for the sub-board disappears under a pathway. We can only assume that it is connected to a buried Earthing Rod. Would it have been better to supply the sub-board with Twin & Earth SWA?

We fed the sub-board via a B16 breaker on the non-RCD side (temporary link) so we could obtain a Ze reading, which was 30 ohms (main consumer unit TNC-S Ze = 0.16 ohms).

We know that a TT system should be a maximum of 21 ohms but we are a little unsure of the 200 ohm limit for a TT system stated in the On-Site Guide.

We did a Zs at a garage socket and obtained a reading of 31.2 ohms. Would this be within tolerance?

RCD’s tripped within the acceptable tripping times. How does this affect the readings as it is tripping two RCDs?

The main source of the tripping was because the UV light for the fish pond had blown and my neighbour was unaware that there was a sub-board in the garage, which when found and switched on, power was re-instated to the garage.

Any advice on the above would be a great help as we have never worked or taken readings on a TT/mixed system before.

 

[trev]

Thomas what is your background and who is the "we"?

 

[D Skelton]

My advice would be to recommend that your neighbour calls a competent and qualified electrician. There are many factors here that need to be taken into account and an electrician is the best person to ascertain whether the current set up is ok or not and also to provide your neighbour with options to remedy the problem/s. If you post your location maybe someone on here may be able to help your neighbour.

 

[Thomas02]

Hi Trev,

We are a small domestic installation company who have been trading for about 6 months or so. Just two of us, both been through college together (2330 Levels 2 & 3) with a few other qualifications - EAL, 17th Edition & PAT Testing and we are NICEIC members.

Taking each job as it comes and still relatively new to the industry, we are encountering problems where sometimes we find a solution ourselves and sometimes we need to seek advice, when people like your good self can hopefully help us out.

Any comments regarding the above would be greatly appreciated.

 

[Knobhead]

Hi Trev,

We are a small domestic installation company who have been trading for about 6 months or so. Just two of us, both been through college together (2330 Levels 2 & 3) with a few other qualifications - EAL, 17th Edition & PAT Testing and we are NICEIC members.

Taking each job as it comes and still relatively new to the industry, we are encountering problems where sometimes we find a solution ourselves and sometimes we need to seek advice, when people like your good self can hopefully help us out.

Any comments regarding the above would be greatly appreciated.

So, by your own admission you really aren’t competent to carry out works for a customer.

The best advice is “walk away”.
A customers installation is not an adventure playground for you to learn in.

 

[Guest55]

whats that i smell ?
ah yes , Electrical Trainee.
;-)

 

[D Skelton]

Hi Trev,

We are a small domestic installation company who have been trading for about 6 months or so. Just two of us, both been through college together (2330 Levels 2 & 3) with a few other qualifications - EAL, 17th Edition & PAT Testing and we are NICEIC members.

Taking each job as it comes and still relatively new to the industry, we are encountering problems where sometimes we find a solution ourselves and sometimes we need to seek advice, when people like your good self can hopefully help us out.

Any comments regarding the above would be greatly appreciated.

Ah, erm, ok...

Well I can't be ar$ed to bash any more 5WWs this week so I will say this and this alone:

My previous advice still stands.

 

[Richard Burns]

My neighbour had problems with her RCD tripping in the main consumer unit and she indicated it may be linked to her fish pond set-up outside the property which is fed from a plug socket in the garage.

We investigated that this particular garage circuit was on a B16. We powered up the breaker but there was no power from the garage. Further investigation led us to find an RCD protected sub-board in the garage.

The feed for the RCD protected sub-board is also RCD protected - is this correct or would it be more practical to move the feed B16 breaker over to the non-RCD side? Depends on the earthing of the SWA armour, but 2 30mA RCD in series is pointless and can cause discrimination problems.

The feed exits the house via an IP rated enclosure and two-core SWA is used for the feed to the sub-board. The main earth for the sub-board disappears under a pathway. We can only assume that it is connected to a buried Earthing Rod. Would it have been better to supply the sub-board with Twin & Earth SWA? I assume you mean 3 core SWA rather than T&E, depending on bonding requirements the SWA may be OK for using the armour as a cpc (but not bonding conductor)

We fed the sub-board via a B16 breaker on the non-RCD side (temporary link) so we could obtain a Ze reading, which was 30 ohms (main consumer unit TNC-S Ze = 0.16 ohms).

We know that a TT system should be a maximum of 21 ohms but we are a little unsure of the 200 ohm limit for a TT system stated in the On-Site Guide. Ignore the 21 ohms and understand the requirements better.

We did a Zs at a garage socket and obtained a reading of 31.2 ohms. Would this be within tolerance? Yes

RCD’s tripped within the acceptable tripping times. How does this affect the readings as it is tripping two RCDs? test each one separately and get both set of readings.

The main source of the tripping was because the UV light for the fish pond had blown and my neighbour was unaware that there was a sub-board in the garage, which when found and switched on, power was re-instated to the garage.

Any advice on the above would be a great help as we have never worked or taken readings on a TT/mixed system before.

I hope this is a free job for your neighbour to help you learn and not paid for because you should be able to manage this if you are doing the work for money. However in the spirit of assuming you are just helping your neighbour with the right test equipment, see my comments above.

 

[Engineer54]

I don't know about the others here, but i just can't believe you or your mate have 2330 2 and 3, not from what you have posted in your OP!!

 

[GLENNSPARK]

I don't know about the others here, but i just can't believe you or your mate have 2330 2 and 3, not from what you have posted in your OP!!

they ought to call themselves BLB electrical...
as in blind leading blind...