Smoke Alarm - Mains, Link and Wiring

[RF Lighting]

You can't prove polarity without an earth

 

[JohnW2]

You can't prove polarity without an earth

True - but it doesn't have to be an earth from the cable in question!

Kind Regards, John

 

[bernardgreen]

If polarity is in doubt then a true ground should be used as reference,

In a worst case error the polarity into the building may be incorrect and the "earthed" CPC is then connected to Live. This would still be safe for anyone inside the equipotential zone inside the building and could go un-noticed if there were no metallic services or other means to bring a true ground potential into contact with the CPC "earth" of the building

 

[JohnW2]

If polarity is in doubt then a true ground should be used as reference, ... In a worst case error the polarity into the building may be incorrect and the "earthed" CPC is then connected to Live.

That's true (but obviously only for TN-C-S) but it seems to represent an extremely deep scraping of the barrel. I suppose that "anything that can happen will happen" (eventually).

However, your point raises an interesting question, to which I don't know the answer - how do DNO workers check/confirm the polarity of incoming TN-C-S supplies? I've never seen them stick in an earth rod for that purpose, but maybe they sometimes do. If there are any bonded extraneous-c-p's, they could use them as their reference but what if there are none - do they then rely on testers which utilise capacitive coupling to earth, or what?

Nor is this totally hypothetical. as I've illustrated in the past, the supply to my house is in PVC/PVC singles and one can see that the neutral has red insulation - which leads me to suspect that the line conductors (whose insulation I can't see) probably also have red insulation ("what was in the van at the time"?).

westie?

Kind Regards, John

 

[Lectrician]

It is also VERY useful when carrying out the tests on the wiring before it is put into service. EFLI cannot be determined unless there is an "E"

That's an interesting, and valid, point. Mind you, if the cable did not have a CPC/earth, then there would not really be an EFLI, in the usual sense, to measure!

Of course, if it were a TT installation, the EFLI would be an irrelevance, anyway

Kind Regards, John

I had an odd one if a warehouse where the fluorescent fittings were fixed directly to the steel purlins with nuts and bolts. The fitting were fed in two core flex from conduit boxes with TRS glands in. The steel conduit also nut and bolted to the steel work. All ripped out now to make way for a cold store.

 

[JohnW2]

I had an odd one if a factory where the fluorescent fittings were fixed directly to the steel purlins with nuts and bolts. The fitting were fed in two core flex from conduit boxes with TRS glands in. The steel conduit also nut and bolted to the steel work. All ripped out now to make way for a cold store.

Interesting, but I somehow doubt that was unique! Returning to TTC's point, did you attempt to measure an EFLI for that circuit?

Kind regards, John

 

[Taylortwocities]

I have a feeling that he was not resposible for the installation of that!

 

[JohnW2]

I have a feeling that he was not resposible for the installation of that!

I don't imagine that he was, but if he came across it in the course of his work, it may well have been appropriate/necessary for him to undertake tests on the circuit!

This is, of course, rather different from the situation which caused you to raise the matter of EFLI measurement, since in Lectrician's case the light fitting did (presumably) have exposed-c-ps with a pretty good earth connection, albeit not via a CPC in the cable! - so I suppose that undertaking an EFLI measurement would not be unreasonable.

Kind Regards, John

 

[westie101]

However, your point raises an interesting question, to which I don't know the answer - how do DNO workers check/confirm the polarity of incoming TN-C-S supplies? I've never seen them stick in an earth rod for that purpose, but maybe they sometimes do. If there are any bonded extraneous-c-p's, they could use them as their reference but what if there are none - do they then rely on testers which utilise capacitive coupling to earth, or what?

The only and I stress only approved use for us of a voltage stick (http://www.screwfix.com/p/lap-ms8902h-ac-voltage-detector-pen/88495) is to prove polarity. We can only purchase specially approved ones from EA Technology http://www.eatechnology.com/products-and-services/field-engineering-tools/polarity-test-kit

The only other approved way is with an earth rod, length of wire and a suitable 2 terminal test lamp, this of course needs to members of staff as one has to "guard" the earth rod

 

[JohnW2]

However, your point raises an interesting question, to which I don't know the answer - how do DNO workers check/confirm the polarity of incoming TN-C-S supplies? I've never seen them stick in an earth rod for that purpose, but maybe they sometimes do. If there are any bonded extraneous-c-p's, they could use them as their reference but what if there are none - do they then rely on testers which utilise capacitive coupling to earth, or what?

The only and I stress only approved use for us of a voltage stick (http://www.screwfix.com/p/lap-ms8902h-ac-voltage-detector-pen/88495) is to prove polarity. We can only purchase specially approved ones from EA Technology http://www.eatechnology.com/products-and-services/field-engineering-tools/polarity-test-kit
The only other approved way is with an earth rod, length of wire and a suitable 2 terminal test lamp, this of course needs to members of staff as one has to "guard" the earth rod

Thanks for clarifying/confirming - much as per my speculations, then!

Kind Regards, John

 

[securespark]

I used to use volt sticks to check metalwork (appliance cases, rads, sinks etc) for live.

 

[JohnW2]

I used to use volt sticks to check metalwork (appliance cases, rads, sinks etc) for live.

Interesting. Was that a practice which was approved by your employer (assuming you had one)? For obvious reasons, it sounds a bit iffy to me - volt sticks aren't too bad when one is trying to confirm that an expected voltage is present (since a 'false' answer is unlikley to result in disaster in that situation), but the concept is much more worrying (at least, to me) if one is trying to confirm that voltage is absent.

Kind Regards, John