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New here. I come from DIYnot. Been a member there for 12 years and i got bored so i came here...

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Question
Ive seen a customer whos got a TT earth with no RCD just MCB's. Is this okay or will it need improving... im there to install downlights so b4 installing need advice.
Cheers

HarlemSparton wrote: ↑Wed Jun 28, 2023 2:02 pm
New here. I come from DIYnot. Been a member there for 12 years and i got bored so i came here...

This forum is quite nice its got nice emojis

Question
Ive seen a customer whos got a TT earth with no RCD just MCB's. Is this okay or will it need improving... im there to install downlights so b4 installing need advice.
Cheers

To be honest if you don't know the answer to this you shouldn't be making changes to the installation .............

Im not come across TT installations much.

Ik tt's wont meet ADS without a form of RCD

From a quick search:-
Higher external earth fault loop impedances are usually associated with TT earthing systems, which don’t provide sufficient fault current to operate fuses or circuit breakers within the required time and an RCD must be used for electric shock protection.
In a TT system MCB's work on any overload or line - neutral/ line - fault as they would in any other system. It is only on an earth fault that they are not effective, so we use RCD's to provide fault protection.

HarlemSparton wrote: ↑Wed Jun 28, 2023 3:09 pm Im not come across TT installations much.

And how do you certificate your changes?

The property looks like a TT but my boss says its a tns. Only two 25mm incoming tails with 10mm's earth on met. Going off to bonds etc Which i disagree its tns. Im a apprentice btw
It might be a tns as why would the previous sparks not include a rcd or a ELCB especially on a TT if it was a older installation.

Ill send a pic of the service head in a bit

This is the incoming
Is this a tns or TT?
Its a tt right but my boss saying its tns cos the sparkies who installed it would never just rely on mcbs on a tt

Unless your boss is a competent spark, what does he know.

What is the Ze?

HarlemSparton wrote: ↑Wed Jun 28, 2023 5:53 pm The property looks like a TT but my boss says its a tns. Only two 25mm incoming tails with 10mm's earth on met. Going off to bonds etc Which i disagree its tns. Im a apprentice btw
It might be a tns as why would the previous sparks not include a rcd or a ELCB especially on a TT if it was a older installation.

Ill send a pic of the service head in a bit

The install May predate the requirement to include RCDs

Neelix wrote: ↑Wed Jun 28, 2023 7:58 pm

HarlemSparton wrote: ↑Wed Jun 28, 2023 5:53 pm The property looks like a TT but my boss says its a tns. Only two 25mm incoming tails with 10mm's earth on met. Going off to bonds etc Which i disagree its tns. Im a apprentice btw
It might be a tns as why would the previous sparks not include a rcd or a ELCB especially on a TT if it was a older installation.

Ill send a pic of the service head in a bit

The install May predate the requirement to include RCDs

For ads it is vital to disconnect the supply if any fault occurs.
What do you think it is? A tt or tns?
Ill take a record of ze and a zdb tomorrow

I have worked on a Radio Hams TT install which had less than one ohm to earth, but only have once worked on such a supply, and since the DNO only say their earth is 15 ohm or better, to get the current required to open a MCB is not going to happen.

I know with my parents house as a boy playing with electrics I managed to rupture the fuse, so at that point there must have been a good earth, but some 30 years latter when I came to change the CU the only earth I could find was the rod fitted by the GPO for the party line phone.

I phoned the DNO asking what earth was fitted, they are obliged to tell you, and they sent out some one to find out, as they had no record, and he gave us TN-C-S. But as to what happened to the original earth I really don't know, likely in 1954 when house built it was from the water main.

I have used the RCD tester on other installations in the past and found it would not trip, step one was measure the earth loop impedance and I was getting a reading of way over 200 ohms, which we are told to consider as unstable, so I went off to get some earth rods, when I came to fit them I found an ELCB-v which explained the high reading.

But as far as your question goes, to fit the down lights you need to complete a minor works certificate, to do that you will need to use the loop impedance meter, and the readings from the meter without a RCD would need to exceed to earth the valve needed to open the MCB/fuse, I will assume MCB, so needs to trip on the magnetic part of the trip, so a B means 3 - 5 times the thermal rating, so a B6 will need 30 amp to trip, (5 x 6) plus 5% for safety so 31.5 amp 230/31.5 = 7.30 Ω, if you can't get 7.3 Ω to earth then improvement is required. If using a C6 then 3.65 Ω etc.

It is unlikely looking at the picture of the head, that you will get a low enough reading.

If you were a DIY guy the answer would have been different, as we know DIY guys often don't have a loop impedance tester, but as an electrician you do, so answer is easy, measure the earth loop impedance.

Not totally sure about the above post but your changes to a lighting circuit would mean mandatory RCD protection for the circuit IMHO

Ramble: My overseas property was TT, the only earth rod being for the shower heater which incorporated an ELCD; when I had an air conditioner fitted, they also put in an earth rod for that; not happy with the situation, I fitted this RCD on the incoming tails to the existing MCB and DP master switch box; would not pass muster in UK though.
On my newer separate extension, I insisted on an RCD type consumer unit; the air con and shower heater still needed separate earth rods.