Double Pole on TT

[hightower]

I'm reading regulation 537.2.1.1 and as far as I can see this states that isolation devices in a TT installation must be double-pole? So, some questions I have:

1) Does this mean that single-pole circuit breakers cannot be used for isolation in a TT system?
2) I'm guessing therefore that the second part of the regulation comes in to force:

Provision may be made for isolation of a group of circuits by a common means, if the service conditions allow this.

So in this case, I'm assuming this would be the main double-pole switch on the board? Which would mean technically you can't isolate a single circuit in a TT system?

3) What is the reason for this requirement of double-pole? If I was to have a guess I'd give this example, although I could be way off with this:

You open a single-pole breaker on a TT system. Working on the lighting circuit, you touch the neutral with one hand, and the earth with your other as you fiddle with the cables. Unbeknownst to you, next door has an unresolved fault and is sending 230v down their earth electrode. This voltage is then coming up your earth electrode, and you've just completed the circuit to the neutral with you in the middle.

That's the reason I think double-pole is required in TT systems, but could somebody clarify?

Thanks,

 

[Richard Burns]

Generally it would be a very good idea to have double pole isolation on each circuit in a TT system but current consumer units do not make this easy.
However the double pole main switch does permit the required double pole isolation.
If you wanted the capability to isolate by switching then double pole switches could be inserted into the outgoing circuits, but it would look a mess.
If you were to also disconnect the neutral from the neutral bar then circuit would be safely isolated, can be useful if you need to avoid tripping a main RCD!

The neutral and the earth may (will) not be at the same potential in a TT system so could present a risk of shock. Pretty much as you have said.