TT: A bit of guidance please

uksparks · Oct 10, 2014

Hi,

Sorry about the quality of the photos, it was very dark in the garage.

Went to a job this morning, to quote on a new consumer unit, see photos below.

It's a TT earthing system, 8 way 3036 board, needs new bonding etc. Main earthing conductor is only 6mm at present, rod is just outside the garage against the wall, no idea how big it is yet.

Its got an overhead supply, two phases, but only one is used.

All the old junk on the right is coming out as its not used, this was on the second phase, it was for storage heaters.

Ill be quite honest, I have never done a TT before so would like a few pointers.

I'm going to replace the board for a split board, thats not an issue, its just the earthing thats the issue.

I did a Zs at the board just to see what the readings were like and you can see the photos below.

A couple of questions...

1. being a TT, i understand it needs an upfront RCD? Yes? 300ma?

2. I took a PSCC and PEFC reading and the PEFC was a lot lower than I had expected and want to know if its normal so to speak, obviously the fault current comes down the higher the resistance.

3. Im permitted 200 Ohms on Ze, so 8.3Ohms is good.

This was with the earth connected, it was only a brief look, so not an exact measurement.

PSCC (L - N)
0.19 Ohms
1.3kA PFC

PEFC (L - E)
8.33 Ohms
30A PFC

Is there anything I should have to think about it addition to the norm with a TNC-S / TNS supply? I can't think of anything other than the RCD.

I felt the 30A was low on the PEFC, but then again, its got an RCD.

[ElectriciansForums.net] TT: A bit of guidance please

I can only assume this was an RCD as it tripped when I did a high current Zs on the board.

Miscellaneous...

guest119 · Oct 10, 2014

PSCC and PEFC are both current, not ohms.

boydy

uksparks · Oct 10, 2014

Boydy said:
PSCC and PEFC are both current, not ohms.

boydy

Sorry yes I know that, it was just how I noted it down, Ill ammend it now.

Octopus · Oct 10, 2014

Use a plastic CU and you won't need an upfront RCD

uksparks · Oct 10, 2014

Murdoch said:
Use a plastic CU and you won't need an upfront RCD

Ok, can you explain why this is please? I was under the impression the RCD upfront was to protect the tails? That sounds wrong doesn't it, sorry having never touched a TT I don't really know the full details.

Octopus · Oct 10, 2014

uksparks said:
Ok, can you explain why this is please? I was under the impression the RCD upfront was to protect the tails? That sounds wrong doesn't it, sorry having never touched a TT I don't really know the full details.

My understanding is that if the tails are surface mounted, and the CU is plastic dual rcds' or RCBO's are OK.

Only done a couple but this was my method both times

trev · Oct 10, 2014

1 S type RCD needed.
2 PEFC will be lower for the reasons you state. Not too sure why you're asking about it mate.
3 8.3 0hms is a lot better than 200 BUT with a bit of effort you can get this to >1ohm
I'll pm you my number so we can have a chat if you want.

telectrix · Oct 10, 2014

the reason for an upfront RCD on a TT system with a metal CU is to trip out the supply in case the tails fault to the CU metal. obviously a 30A PEFC will not take out the OCPD. that "RCD" is a VOELCB. piece of junk.

HandySparks · Oct 10, 2014

uksparks said:
2. I took a PSCC and PEFC reading and the PEFC was a lot lower than I had expected and want to know if its normal so to speak, obviously the fault current comes down the higher the resistance.

PEFC (L - E)
8.33 Ohms
30A PFC

Check the calc for yourself. The values you show imply a measured supply voltage of 250V. So a Zs of 8.33 ohms will give an earth fault current of 8.33A. (I=V/R)

uksparks said:
3. Im permitted 200 Ohms on Ze, so 8.3Ohms is good.

This was with the earth connected, it was only a brief look, so not an exact measurement.

You need to re-measure the rod alone. It's likely that the installation is being 'earthed' by gas or water pipework and that the earth fault loop resistance via the rod will be considerably higher.

telectrix · Oct 10, 2014

HandySparks said:
Check the calc for yourself. The values you show imply a measured supply voltage of 250V. So a Zs of 8.33 ohms will give an earth fault current of 8.33A. (I=V/R)

You need to re-measure the rod alone. It's likely that the installation is being 'earthed' by gas or water pipework and that the earth fault loop resistance via the rod will be considerably higher.

250/8.33 = 30.012A ( i'm sure your post was a typo. LOL.)

uksparks · Oct 10, 2014

Thanks for the pointers here, I'll have a look in the book (GN8) too to see what else it says.

I have got the job on the basis my quote is around the figure I mentioned so may look at investing in some leads so that I can properly test the rod in it's own.

im sure you are right about the water bringing the reading down, I'm going back at some point next week to test it all and see what readings I get in the circuits etc, so will disconnect the rod and get an accurate reading from it.

as for the upfront rcd, it would be a plastic CU so maybe I don't need it then, what you are saying sounds logical.

davesparks · Oct 10, 2014

The first thing I would do is have a little walk down the street and see if PME is available in the area, you only have to look at the poles holding the overheads up.

What do you think is wrong with a 6mm Earthing conductor?

You use a 100mA S type RCD as a main switch to give additional fault protection of you cannot meet the Zs requirements for the circuits. I believe it is necessary regardless of the use of RCDs elsewhere as they are such unreliable devices that it gives a better chance of at least one operating in the event of a fault.

Who has permitted you a Ze of 200ohms? This figure is incorrectly thrown around as a limit for Ra,

I've got to say its nice to see that the NICEIC haven't lowered their standards for approved contractors, it would be terrible if they accepted people who don't even know what they are doing with a TT system wouldn't it!

HandySparks · Oct 10, 2014

telectrix said:
250/8.33 = 30.012A ( i'm sure your post was a typo. LOL.)

Well spotted, tel.
30A was indeed the figure I should have quoted.

uksparks · Oct 10, 2014

davesparks said:
The first thing I would do is have a little walk down the street and see if PME is available in the area, you only have to look at the poles holding the overheads up.

What do you think is wrong with a 6mm Earthing conductor?

You use a 100mA S type RCD as a main switch to give additional fault protection of you cannot meet the Zs requirements for the circuits. I believe it is necessary regardless of the use of RCDs elsewhere as they are such unreliable devices that it gives a better chance of at least one operating in the event of a fault.

Who has permitted you a Ze of 200ohms? This figure is incorrectly thrown around as a limit for Ra,

I've got to say its nice to see that the NICEIC haven't lowered their standards for approved contractors, it would be terrible if they accepted people who don't even know what they are doing with a TT system wouldn't it!

Cheeky git! Living on the edge of a city, TT is not something you tend to see much of!

200 Ohms, you know as well as I do where that came from.

Ill stick a 100mA upfront, but its going to trip and knock everything out isnt it when there's a fault? Sutrely a 300mA would be more suitable in that respect?

extec · Oct 10, 2014

Ill stick a 100mA upfront, but its going to trip and knock everything out isnt it when there's a fault? Sutrely a 300mA would be more suitable in that respect?[/QUOTE]

Not if it is S type in fact its not necessary OTT to fit an up front RCD.