Just a wild thought - could it be a 'hand-down' from some industrial 3-phase installations which, as I understand it, do not necessarily distribute the neutral (for balanced 3-phase loads)?
Kind Regards, John.
Anyone for a widowmaker ?
- Thread starter SimonH2
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Kind Regards, John.
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Well it's true that some 3P equipment doesn't need a neutral, and there are installation that only provide a "3 wire" supply.
That doesn't explain why for 1P installations we seem to require 2 pole isolation in the main switch, while in 3P+N installations we don't require a 4 pole main switch.
If it's dangerous to leave the neutral connected on a 1P system, why isn't it dangerous to leave it connected in a 3P system ? After all - in many installations you'll have 3P into the building, and then a number of 1P boards for 'domestic' loads. So for one circuit, you could be working with the neutral connected or isolated depending on where the supply was knocked off
That doesn't explain why for 1P installations we seem to require 2 pole isolation in the main switch, while in 3P+N installations we don't require a 4 pole main switch.
If it's dangerous to leave the neutral connected on a 1P system, why isn't it dangerous to leave it connected in a 3P system ? After all - in many installations you'll have 3P into the building, and then a number of 1P boards for 'domestic' loads. So for one circuit, you could be working with the neutral connected or isolated depending on where the supply was knocked off
I agree that if there is any distribution of neutral (i.e. 4-wire+CPC wiring, or 1-phase+N+CPC circuits fed from the system), then your question is valid. However, I was suggesting that the practice of 3P (rather than 3P+N) may have arisen from industrial installations with a "3 wire supply".
Another thought, in terms of the regs. Isolation of neutral is neutral is, in general, not required for TN installations, except that 537.1.4 requires that, specifically for 1-phase supplies, a main switch to be used by ordinary persons (e.g. in domestic CU) must isolate both L and N.
Kind Regards, John.
We never specifically isolate the neutral, in fact we are not permitted to.
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The only time you need to isolate the neutral on a three phase install is on a TT installation. While 4 pole incomers are available for most boards, it is extremely common to only use TP incomers.
With a TP generator, the changeover switch should always be 4 pole.
A rod should always be connected to the generator, but there is no need to segregate the earthing from the supplies earth, as long as you provide your own too (The generator should have a N to E link and this should be bonded to a rod or tape etc). Quote common to see hard drawn bare copper conductors buried in a trench for use as the earth on larger installs.
With a TP generator, the changeover switch should always be 4 pole.
A rod should always be connected to the generator, but there is no need to segregate the earthing from the supplies earth, as long as you provide your own too (The generator should have a N to E link and this should be bonded to a rod or tape etc). Quote common to see hard drawn bare copper conductors buried in a trench for use as the earth on larger installs.
That is what has been observed, and the question was why this differed from the almost universal practice with single-phase boards. I have offered some possible explanations. As I said, it seems that, in addition to TT installations, the crunch is probably 537.1.4 which effectively specifically requires a DP main switch for all domestic (plus many other) single-phase installations (including TN ones).
Kind Regards, John.
I don't think anyone has clearly got to the bottom of why three phase TN systems are not required to have neutral isolation where as single phase supplies (including those derived from the same three phase supply) do.
It has been discussed quiet often, and never a clear answer given.
The reg is there though, clear as crystal.
It has been discussed quiet often, and never a clear answer given.
The reg is there though, clear as crystal.
Indeed. Given the wording of 537.1.4, maybe someone thought that 'ordinary persons' would not commonly be operating main switches of 3-phase installations?
Kind Regards, John.
That's certainly the way I understand it, but I think it's one of those mysteries of our industry!
Maybe it's beacuse it's much harder to end up with reverse polarity on a TP&N supply than it is on an SP supply?
Maybe it's beacuse it's much harder to end up with reverse polarity on a TP&N supply than it is on an SP supply?
All I can think of is that with a TP & N switch there is an increased risk of loss of neutral. As we know a lost neutral on an unbalanced system can get "nasty".
In fact for TP&N it could be sensible for the neutral to make first & disconnect last.
Whereas if only TP switching is involved the neutral integrity would be more secure.
Obviously for a changeover switch the increased risk is probably outweighed by any risk of having a generator & DNO neutral commoned
For single phase, loss of neutral on a main switch has no major dangerous implications.
In fact for TP&N it could be sensible for the neutral to make first & disconnect last.
Whereas if only TP switching is involved the neutral integrity would be more secure.
Obviously for a changeover switch the increased risk is probably outweighed by any risk of having a generator & DNO neutral commoned
For single phase, loss of neutral on a main switch has no major dangerous implications.
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