Assistance with my installation

[Poacher3536]

I’m a trainee electrician and would really appreciate the benefit of someone’s skill and experience on the following:

Our house is a TN-C-S (100A fuse) with a main DB (100mA RCD).

This DB feeds a sub DB in a garage outbuilding via a 6mm SWA with all 3 conductors used, by a 63A MCB. The armour is also earthed for mechanical protection.

The sub DB (30mA RCD) is also a TT system with 10mm earth cable to electrode. (I’ve tested the electrode and it’s at 83 Ohms so well inside the 200 Ohms limit.

The sub board feeds the following in garage and outside:

Outside

32A hot tub via 6mm SWA

16A power radial

6A lighting

Inside garage:
32A ring

6A lighting

I’ve checked all cable runs for voltage drop and current capacity, all fall within BS7671.

The issues I’m unsure of is:

1.The sub DB is taking the PME from main DB, whilst also benefiting from an earth electrode. AFAIK having 2 x systems of earthing may conflict and lead to main DB RCD tripping, and not the sub board, but I could be wrong?

2.The 6mm SWA has a current capacity of 53A (ref method D), with a 32A hot tub and diversity applied to both power and lighting I’m coming in at 36.92A + 32A = 68.92A. In reality the ring circuit in garage is rarely used above a freezer and occasional drill etc. Certainly nothing with a load that would require a ring final circuit for a new instal.

My initial thoughts were to change the ring final to a radial, with diversity applied will be 13A therefore bringing the total to 60.9A but this is still too much. The next choice would be to upgrade the feed SWA to 10mm giving me 71A.

What’s your options on this? And the best way to make things compliment, both for SWA size and earthing issue?

Thanks in advance.

 

[Pretty Mouth]

Hi @Poacher3536 ,

The 6mm SWA has a current capacity of 53A (ref method D)

Yes, but probably no. Take a look at note 1 of table 4E4A, on page 417 of the blue book. Then see table 4D4A on page 407

 

[Dartlec]

Couple of questions:

Is the outbuilding separate from the house, or attached? If separate are there any extraneous conductive parts? (water feed, metal structure, etc)

Is there a particular reason to want to 'make it compliant', or just for your own peace of mind?

There is a useful guide here on exporting PME to a remote building. It's not prohibited in BS7671, though some DNOs forbid it, and even if they don't there are often issues to consider re safety.

It sounds like as it is set up, then the sub DB is not a TT system, it's an exported PME with an additional earth rod (which might in fact become the norm in future regulations as it happens).

That doesn't necessarily cause an issue with RCD tripping - you probably have enough difference between the 100mA at the source and the 30mA at the garage to ensure selectivity, such that the garage trips before the main house.

Is the 100mA at the house end only protecting the SWA, or the whole house as well? Has there ever been problems with it tripping? SWA properly installed wouldn't necessarily need an RCD at the front end, just a suitable MCB.

To make the sub db a TT install, the armour of the feeding SWA would have to be isolated (usually in a plastic glanding box) to ensure it is NOT connected to the sub db or install (plus any core that might be connected as CPC of course).

Given that the sub db feeds a hot tub, making it a TT may well be the way to go - there are some good previous answers on here re hot tub install, as well as a very good video by David Savery on the issues involved...(not necessarily suitable to watch with children or the easily offended around)

Re the final issue with the SWA current capacity, it is likely not a huge issue, but a simple solution if it concerns you would seem to be to fit a 50A MCB at the house end to protect the SWA (Edit - but see response by @Pretty Mouth above first)

In practise, it seems very unlikely that even with a hot tub, you would ever actually pull anything over that in reality. What is the 16A radial feeding?

Given all this is already installed, there is no need necessarily to bring it up to current spec in any case, unless there is anything that is dangerous...

Of all the issues, only the exported PME seems like a possible issue to deal with to me, and in that case only because of the hot tub.

 

[Lucien Nunes]

1. If the garage earthing is connected to the supply TN-C-S earth, it's not TT. To make it TT, it must be disconnected from that earth so that the garage DB earth bar takes up the earth electrode potential. Otherwise, at 83 ohms, compared to a fraction of an ohm from the TN-C-S, the electrode has negligible influence on the system.

Two systems of earthing can't 'conflict,' merely the one that has lower Ze will dominate and control the potential at the MET. Nor will they have any influence on RCDs, which only measure differential current in L & N. The RCD has no connection to, nor any sense of, the earthing system. The RCD doesn't even 'know' whether there is an earthing system. Historically, in the days of VOELBS which sensed currents in the system CPCs, this situation was more complex and separate earthing systems could interact. But that doesn't apply in any way to RCDs.

What is of interest is whether there is selectivity between the two RCDs. Why is there a 100mA upfront on a TN-C-S?

2. When calculating design loading, the circuit configuration itself is of little account, unless it tells you about the size and usage of the installation. A big house with ten rings is likely to have a higher total load than a small house with two. But if you fit ten rings in the small house, you still only have a small house load, so you can't use the normal diversity values for that case because it's not representative. Likewise If you put a 32A ring in a garage with just a freezer and a few chargers, you still only have a freezer and a few chargers. It matters not whether the circuit is a ring or a radial.

Work out what the load is in reality. Is the hot tub load actually 32A? Read the data plate. Add it to a realistic load from the garage etc. Someone might plug in a heater, it could be 10A for that, 5A for a power tool, 2A for the freezer starting current and 1A for lighting. Or something like that. If the result is under 50A, put a 50A MCB in the main board to protect the cable that has an Iz of 53A, and Bob's your circuit.

BTW worth getting used to working in the correct precision for the job in hand:

I’m coming in at 36.92A + 32A = 68.92A

69.92A means more than 68.915A but less than 69.925A. Those four sig fig are implying you know the result to within 10mA, when in reality it's probably wrong by 20A. I'd write 69A, but think 'around 70A.'

Simulpost with Dartlec making a lot of the same points.

 

[Pretty Mouth]

though some DNOs forbid it

I don't believe this to be true @Dartlec, I believe this is a (rather persistent) myth .

I have it in writing from 3 DNOs (every one that I have contacted so far) that they have no special requirements regarding earthing arrangements at outbuildings.

 

[davesparks]

I’ve checked all cable runs for voltage drop and current capacity, all fall within BS7671.

No they don't, you've got 6mm protected by a 63A MCB, that's almost certainly not compliant.

The sub DB is taking the PME from main DB, whilst also benefiting from an earth electrode. AFAIK having 2 x systems of earthing may conflict and lead to main DB RCD tripping, and not the sub board, but I could be wrong?

With the submain CPC connected at the second DB you don't have 2 systems of earthing, you only have one, TNCS, it just happens to have an earth electrode connected within the installation.

Regardless of what is done with the earthing the RCD at the first DB may trip on faults at the second DB, RCDs work on current imbalance.

My initial thoughts were to change the ring final to a radial, with diversity applied will be 13A therefore bringing the total to 60.9A but this is still too much.

Changing the ring to a radial will achieve nothing, the load won't change.

What’s your options on this? And the best way to make things compliment, both for SWA size and earthing issue?

Two options for dealing with the cable size issue are either a bigger cable or a smaller OCPD.

What earthing issue do you have? Is there any reason why you would need to second DB to have its own seperate TT earthing system?

 

[Dartlec]

I don't believe this to be true @Dartlec, I believe this is a (rather persistent) myth .

I have it in writing from 3 DNOs (every one that I have contacted so far) that they have no special requirements regarding earthing arrangements at outbuildings.

Which 3 out of interest?

It may well be a myth - or there might be one somewhere, since it always seems to come up in articles/guides on the issue.

Of course, if something were to happen and the HSE were involved, they might claim something different in that specific case, so it's good to have it in writing.

I guess it would come down to what risk assessment was done at time of install.

 

[davesparks]

though some DNOs forbid it, and even if they don't there are often issues to consider re safety.

No they don't forbid it, they just expect you to do it in a safe and compliant manner.

DNO's will not supply a TNCS earth connection to various types of installation due to the possible dangers, but the average garden building isn't on that list.

 

[Poacher3536]

Hi @Poacher3536 ,


Yes, but probably no. Take a look at note 1 of table 4E4A, on page 417 of the blue book. Then see table 4D4A on page 407

Thank you, I’ve only got the brown book but looked up the tables. Basically the cable is de-rated to 46A so it operates at 70 instead of 90 degrees?

 

[Pretty Mouth]

Which 3 out of interest?

It may well be a myth - or there might be one somewhere, since it always seems to come up in articles/guides on the issue.

Of course, if something were to happen and the HSE were involved, they might claim something different in that specific case, so it's good to have it in writing.

I guess it would come down to what risk assessment was done at time of install.

WPD, UKPN, and Electricity North West.

I contacted them after a discussion on a FB group where someone claimed that UKPN told him on several occasions that they prohibited exporting the TN-C-S earth to outbuildings such as garages, sheds etc. I suspected he wasn't telling the truth, so I both phoned and emailed UKPN, and it turns out he was indeed talking out of his other mouth.

I intend to contact the other DNOs on the matter so that I can say 100% one way or the other. I'll post all their respones in a thread when I do

 

[Pretty Mouth]

Thank you, I’ve only got the brown book but looked up the tables. Basically the cable is de-rated to 46A so it operates at 70 instead of 90 degrees?

Yes, you got it

 

[mainline]

I don't believe this to be true @Dartlec, I believe this is a (rather persistent) myth .

I have it in writing from 3 DNOs (every one that I have contacted so far) that they have no special requirements regarding earthing arrangements at outbuildings.

We do have to remember though that bs7671 forbids the use of pme earthing for some types of installations.

 

[Poacher3536]

Thank you everyone for your input, it’s much appreciated. I will respond later on with a proposal to remedy the situation based on your feedback and my research on points you’ve made (I’d rather learn from this by getting the books out too)

 

[Pretty Mouth]

We do have to remember though that bs7671 forbids the use of pme earthing for some types of installations.

Your point is...?

 

[mainline]

Your point is...?

You said, "I have it in writing from 3 DNOs (every one that I have contacted so far) that they have no special requirements regarding earthing arrangements at outbuildings."



Because a diyer/trainee reading that statement may well believe that they can export pme to any outbuilding without any special requirements, which isn't true.