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Over the last few days, I have stumbled across a number of threads that seem to point to a prevalent misunderstanding. Either that, or I have been misunderstanding the intention of the posters. I'm not linking any threads here as I don't want to call anyone out for this specifically.
In each case, the discussion runs along the following lines:
OP: I get continuity / zero IR between some neutral and some CPC somewhere. Please help.
Helpful poster: Were they still attached to the bars in the CU? Was the main switch on? Is the supply TN-C-S?
The crux is that third question. By specifically asking it, or some variant of it, the helpful poster is implying that TN-C-S supplies have solid continuity between N & E, and by extension, that other configurations do not, including TN-S.
By definition, all TN systems have solid continuity between N & E; the only difference between TN-S and TN-C-S in that regard is where the nearest connection is located. In TN-C-S it is at the head, in TN-S it could be as far away as the substation, but there will be a metal conductor between the two somewhere. TT has a rather less substantial connection via the rod, so you've got Ra in the test circuit which will likely swamp the resistance of the copper, but 'in spirit' the two are still connected.
To an insulation tester, such differences in resistance of a few hundred ohms are neither here nor there. Even a TT system with terribly high Ra still has good enough continuity between N & MET to read 0.0 megohms between the CU busbars as soon as the main RCD is flipped on. More interesting is what you read on a continuity test in the same place. Clearly, TN-C-S is going to read near zero ohms because the two conductors are joined right there on the spot. Supplies delivered as TN-S but locally PME'd in the road are also likely to do so. In theory even TN-S supplies with the whole length of distributor cable between head and substation in-circuit are not going to read very much resistance, still zero for practical purposes.
The reason one might not get a near-zero reading N-E of a TN-S supply is the voltage drop along the neutral caused by load current. The low test voltage applied by a continuity range tends to get swamped by the AC voltage drop, causing the tester to display a reading that could be anywhere between correct and nonsense, even negative. The resistance of the N-E path is still low, it's just that the continuity tester can't measure it due to the interference. The IR tester with its much higher voltage and lower resolution is less sensitive and likely to return a consistent zero reading.
Now I've got that off my chest, I'll also mention that I have seen a couple of mentions of 'buzz bars' recently too. The 'bus' in 'busbar' is short for omnibus, just like the big vehicle with lots of seats, i.e. a means of transport available to everybody (or every circuit.) Do people really take a ride on a 'buzz' or what? Or do they think the bars make a buzzing noise?
Full disclosure: I'm lying in a hospital bed with lots of background noise completely unable to get to sleep. If it carries on like this tomorrow I'll find some more niggles to whinge about.
In each case, the discussion runs along the following lines:
OP: I get continuity / zero IR between some neutral and some CPC somewhere. Please help.
Helpful poster: Were they still attached to the bars in the CU? Was the main switch on? Is the supply TN-C-S?
The crux is that third question. By specifically asking it, or some variant of it, the helpful poster is implying that TN-C-S supplies have solid continuity between N & E, and by extension, that other configurations do not, including TN-S.
By definition, all TN systems have solid continuity between N & E; the only difference between TN-S and TN-C-S in that regard is where the nearest connection is located. In TN-C-S it is at the head, in TN-S it could be as far away as the substation, but there will be a metal conductor between the two somewhere. TT has a rather less substantial connection via the rod, so you've got Ra in the test circuit which will likely swamp the resistance of the copper, but 'in spirit' the two are still connected.
To an insulation tester, such differences in resistance of a few hundred ohms are neither here nor there. Even a TT system with terribly high Ra still has good enough continuity between N & MET to read 0.0 megohms between the CU busbars as soon as the main RCD is flipped on. More interesting is what you read on a continuity test in the same place. Clearly, TN-C-S is going to read near zero ohms because the two conductors are joined right there on the spot. Supplies delivered as TN-S but locally PME'd in the road are also likely to do so. In theory even TN-S supplies with the whole length of distributor cable between head and substation in-circuit are not going to read very much resistance, still zero for practical purposes.
The reason one might not get a near-zero reading N-E of a TN-S supply is the voltage drop along the neutral caused by load current. The low test voltage applied by a continuity range tends to get swamped by the AC voltage drop, causing the tester to display a reading that could be anywhere between correct and nonsense, even negative. The resistance of the N-E path is still low, it's just that the continuity tester can't measure it due to the interference. The IR tester with its much higher voltage and lower resolution is less sensitive and likely to return a consistent zero reading.
Now I've got that off my chest, I'll also mention that I have seen a couple of mentions of 'buzz bars' recently too. The 'bus' in 'busbar' is short for omnibus, just like the big vehicle with lots of seats, i.e. a means of transport available to everybody (or every circuit.) Do people really take a ride on a 'buzz' or what? Or do they think the bars make a buzzing noise?
Full disclosure: I'm lying in a hospital bed with lots of background noise completely unable to get to sleep. If it carries on like this tomorrow I'll find some more niggles to whinge about.
