Testing questions?

[Colin33]

I haven't done any testing for a long time so some help with a few questions muchly appreciated please!

Firstly, earthing. The supply cable (from underground) appears to have several strands of the sheathing coated in black insulation and which is used as the neutral with the remaining bare sheathing as earthing. I've not seen this before, only the usual TNCS where the whole of the sheathing is the combined earth/neutral. Does this mean I have a TNS system? BTW, the Ze measurement is 0.16

Secondly, this is an all RCBO installation. However, when testing the trip times of all circuits generally come up with the same reading of 19ms, even on the fast trip test. There were a few minor variations when switching test between 0 & 180 degrees but 19ms was the favourite reading! I'm presuming that these particular rcbo's won't trip any faster than this?

Also, all circuits have a combined fixing method in that parts of the wiring are clipped direct to timber above the loft insulation, then drop into conduit in plastered walls and/or through insulated stud walls, and sometimes into plastic trunking and conduit in places. Which 'reference method' should one choose to put on the paperwork in this instance?

Cheers....

 

[IQ Electrical]

I haven't done any testing for a long time so some help with a few questions muchly appreciated please!

Firstly, earthing. The supply cable (from underground) appears to have several strands of the sheathing coated in black insulation and which is used as the neutral with the remaining bare sheathing as earthing. I've not seen this before, only the usual TNCS where the whole of the sheathing is the combined earth/neutral. Does this mean I have a TNS system? BTW, the Ze measurement is 0.16

Secondly, this is an all RCBO installation. However, when testing the trip times of all circuits generally come up with the same reading of 19ms, even on the fast trip test. There were a few minor variations when switching test between 0 & 180 degrees but 19ms was the favourite reading! I'm presuming that these particular rcbo's won't trip any faster than this?

Also, all circuits have a combined fixing method in that parts of the wiring are clipped direct to timber above the loft insulation, then drop into conduit in plastered walls and/or through insulated stud walls, and sometimes into plastic trunking and conduit in places. Which 'reference method' should one choose to put on the paperwork in this instance?

Cheers....

1 The cable is concentric and the earthing system is usually TN-C-S (concentric is used on TN-C-S supplies to lessen the chance of broken PEN conductors)

2 It's unusual to have such consistent tripping times but assuming confidence in your test instrument, that's what it's reading.

3 Record the worst-case reference method then you're always covered!

 

[Colin33]

1 The cable is concentric and the earthing system is usually TN-C-S (concentric is used on TN-C-S supplies to lessen the chance of broken PEN conductors)

2 It's unusual to have such consistent tripping times but assuming confidence in your test instrument, that's what it's reading.

3 Record the worst-case reference method then you're always covered!

TNCS systems I've seen before have not had a few strands of the sheath with black insulation, that's what's confusing me. Also, there is usually a PME warning sticker put in place by the DNO but there isn't one in this case?

If I record the worst-case reference method then wouldn't questions be asked as to why I haven't applied the relevant correction factors to the circuits?

 

[IQ Electrical]

The designer would assume responsibility for that, I'm assuming this is initial verification?

 

[SirKit Breaker]

2 It's unusual to have such consistent tripping times but assuming confidence in your test instrument, that's what it's reading.

And 19mS is bloody fast, have a look at the requirements of 415.1.1

Cheers............Howard

 

[Colin33]

The designer would assume responsibility for that, I'm assuming this is initial verification?

This is a complete rewire so a new installation cert. And so yes, I'm also the 'designer'

 

[Colin33]

2 It's unusual to have such consistent tripping times but assuming confidence in your test instrument, that's what it's reading.

And 19mS is bloody fast, have a look at the requirements of 415.1.1

Cheers............Howard

Yes it's well within requirements, but usually get around 12ms with the regular rcd's (as opposed to these rcbo's)

 

[Engineer54]

Firstly, earthing. The supply cable (from underground) appears to have several strands of the sheathing coated in black insulation and which is used as the neutral with the remaining bare sheathing as earthing.

This is the modern DNO cable for TN-S, it's called a ''Split Concentric cable''. It is basically the same as the TNC-S/PME concentric cable, but has, as you have stated, insulated neutral conductors (strands) and bare earthing conductors strands forming the concentric sections.

 

[Colin33]

This is the modern DNO cable for TN-S, it's called a ''Split Concentric cable''. It is basically the same as the TNC-S/PME concentric cable, but has, as you have stated, insulated neutral conductors (strands) and bare earthing conductors strands forming the concentric sections.

Ah, thanks for that. So it is a TNS supply rather than a TNCS then? The Ze seems rather good for a TNS supply though??