TN-S or TN-C-S?

[manda]

Whilst having a quick nosy at my mum's supply I came across a recent installation certificate that was left when her new central heating system was fitted. Before reading the certificate, I was of the opinion it was a TN-C-S supply. However, according to the certificate, they noted it as a TN-S supply with A Ze of 0.22. So I thought I'd stick a few photo's up and see what you guys think as I'm now a tad confused

 

[topquark]

I can't tell from those pics.

 

[telectrix]

apart from being RAF, how is the main earth connected. ? TN-S is on the incoming metallic sheath of the supply cable. TNC-S is linked to the neutral in the cut-out.

edit. looks like the thin bare wire to the metal box is the main earth. does that box house the DNO fuse? if so, it looks like TN-S

 

[IQ Electrical]

Very difficult to tell, was it a TT system previously?

If so then it's likely to be a TN-C-S try an earth loop and neutral loop and see if the results are the same, that will almost confirm TN-C-S.

 

[giio]

it,s a tns supply, the supply cable is the pyro and the earth to the earth terminal block is coming from the casing of the housing, I would also add that it could do with an upgrade,

does the supply come from a pole mounted transformer

 

[telectrix]

it,s a tns supply, the supply cable is the pyro and the earth to the earth terminal block is coming from the casing of the housing, I would also add that it could do with an upgrade,]
that's just what i thought. upgrade to current type head

 

[SPARTYKUS]

Hi Manda

where abouts is that out of interest?

Never seen one like that before

 

[malcolmsanford]

Is this a flat Manda ? As I would say that your riser cable is MICC/Pyro, as that looks to me to be a pyro terminating into that white header box, which as a seal on it, and most likely as a BS 1361 fuse inside it.

You also seem to have an upfront RCD from the meter before it goes into the CU, so I dont think it will be a TNC-S. From that white header box you look to have a really small 1.5mm earth to the BMT, which should be changed as its unprotected and not mechanically protected.

Now becasue you have an RCD there, by the way what size is it 100mA or 30mA, as you also seem to have an RCD in the CU. I think what as happened , the house will have a TN-S supply and they may not be able to achieve good Zs results on the final circuits and so fitted an upfront RCD providing protection.

Now if you mums is a flat you will have to find where that Ze was taken, was it at the origin where it should be, or was it at your mums board where it would be a Zdb. If it was at your mums board and it is a flat then that 0.22 is quite easily got due to parallel paths, can you give us a little more info as I might be well out lol

 

[manda]

There is no earth to the metalic cable, at least not on the outside of the cut out. It is sealed and I don't fancy breaking the seal just to nosy. As for testing it, there is soon to be some work done, so I'll get it tested then out of interest. I don't know if it was previously a TT system, she's only been in there 5 years so all I can say is not in that time. Just wondered if I was missing something that was visually obvious without breaking the seal. Also, would a Ze of 0.22 be acceptable for TN-S as I thought the Maximum was 0.8 thus making it more likely to be a TN-C_S?

 

[IQ Electrical]

Is it actually an MICC supply cable?

 

[aptsys]

You also seem to have an upfront RCD from the meter before it goes into the CU, so I dont think it will be a TNC-S. From that white header box you look to have a really small 1.5mm earth to the BMT, which should be changed as its unprotected and not mechanically protected.

Now becasue you have an RCD there, by the way what size is it 100mA or 30mA, as you also seem to have an RCD in the CU.

There's no RCD in the CU, so the upfront RCD was probably just installed to provide RCD protection rather than anything to do with Zs

 

[topquark]

That Ze is acceptable for both.

 

[topquark]

Is it actually an MICC supply cable?

I'm not sure.

 

[aptsys]

Supply is clearly pyro as giio said, and earth taken from the sheath of the pyro, so not TT. Is this a submain?

 

[manda]

Yes it's a flat in sunny Rochdale. I was also a little perplexed at the lovely unsheathed earth wire. Not a clue where the readings were taken as I wasn't there when they installed it. The RCD is 30mA if I remember right, I'm home again now so can't check, but pretty sure it was 30mA.

 

[malcolmsanford]

There's no RCD in the CU, so the upfront RCD was probably just installed to provide RCD protection rather than anything to do with Zs

Well I'll have to take your word for that as you most likely have younger eyes than me, and that is why I said seem to have as I wasnt sure ...................thank you

 

[telectrix]

my eyes ain't so good either , malc. could be the beer though.

 

[manda]

Type of cable supplying...pass, I've not got that far yet in my study. It is metalic if that helps.

 

[aptsys]

Well I'll have to take your word for that as you most likely have younger eyes than me, and that is why I said seem to have as I wasnt sure ...................thank you

I recognise the wylex CU and the main switch The 2 way CU does have an RCD though.

As it is a submain, you can't determine the type of supply without checking the origin.

 

[malcolmsanford]

As we say its MICC and your getting the earth via the sheath and then onto your BMT via that small earth. You are going to have to find out at the origin of the supply, ie where the DNO main head is to get your Ze, what you will most likely find is that the Ze on your schedule of results are a Zdb from your mums board, if it was then it should be mentioned as such on the EIC, as I would guess they would not have arranged to disconnect the earth conductor at origin, as it would mean isolating other flats and communal area.

 

[manda]

Cheers all, Seems I've managed to gain some more knowledge regards types of supply then, so Thanks for the input. Much appreciated

 

[giio]

There is no earth to the metalic cable, at least not on the outside of the cut out. It is sealed and I don't fancy breaking the seal just to nosy. As for testing it, there is soon to be some work done, so I'll get it tested then out of interest. I don't know if it was previously a TT system, she's only been in there 5 years so all I can say is not in that time. Just wondered if I was missing something that was visually obvious without breaking the seal. Also, would a Ze of 0.22 be acceptable for TN-S as I thought the Maximum was 0.8 thus making it more likely to be a TN-C_S?

the pyro copper sheath is the earth,it is bonded to the metal case by the gland, the earth is then taken from this metal case, used to be common in the old days,but the loads were nowhere near as much as they are nowadays, nor the regs as stringent
0.22 is acceptable for tn-s and tn-c-s maximums are 0.8 for tn-s and 0.35 for tn-c-s

 

[IQ Electrical]

the pyro copper sheath is the earth,it is bonded to the metal case by the gland, the earth is then taken from this metal case, used to be common in the old days,but the loads were nowhere near as much as they are nowadays,or the regs as stringent
0.22 is acceptable for tn-s and tn-c-s maximums are 0.8 for tn-s and 0.35 for tn-c-s

But if this is a distribution circuit then we're all just guessing the system type!

 

[topquark]

Purzactly, I need a beer!

 

[telectrix]

too right. who knows what's in the black magic box ( or in this case off white.)

 

[telectrix]

Purzactly, I need a beer!

i'll join you, not that i'm implying that you're coming apart.......

 

[topquark]

i'll join you, not that i'm implying that you're coming apart.......

What's the saying? I'm not a complete idiot, I have bits missing!

 

[Des 56]

They were originally fitted as a package,the head containing the fuse and the metal clad dist board
The bolt on the casing normally provides the connection to a Tns system,earth cable bolted to that section
Before the old metal clad was removed it would have been directly bolted to the head and the tail you see was also linking both

In another thread I stated a TT system need have no RCd before the tails where precautions have been taken against them shorting to the metal,in this case, it seems the Rcd should also protect the tails

To the question,every one of these were probably original Tns and with there being no change of the head I would say its Tns

 

[Des 56]

i'll join you, not that i'm implying that you're coming apart.......

I'm having a few beers in a little while,I must be running late and have the feeling that the party has already begun

 

[manda]

I managed a few beers to calm my shattered nerves earlier, after watching my mum try out her new disability scooter. She will be getting insurance for it, including 2 million squids of public liability, me thinks she may need it!