On Thursday, 16 November 2017 08:58:00 UTC, John Rumm wrote:
On 15/11/2017 17:28, Andy Burns wrote:
whisky-dave wrote:

John Rumm wrote:

If the voltage sags too far, then you get lower PSCC so can expect
slower operation of protective devices under fault conditions, so
increase electrocution risk, and catastrophic cable failure risk.

But not in under 3 hours.

The 3 hours at 40+ Amps is the "slow" part of the MCB curve, in the
event of a short circuit you want sufficient current to ensure it trips
within the "fast" part of the curve ...

Indeed...

You need 160A to make sure a B type MCB will trip in the magnetic part
of its response curve and that implies a maximum loop impedance at the
point of the fault of 230 / 160 = 1.44 ohms [1]

So why did it trip out ?


Even if the circuit is in spec and meets that requirement at all
sockets, running at only 202 volt gives a reduced potential worst case
PSSC of 202 / 1.44 = 140A, which could leave you on the thermal portion
of the response curve and *20 seconds* away from disconnection. (and
that ignores the effect of elevated conductor temperature on the loop
impedance)

140A I don't think we were drawing anywhere near that. Is that the sort of current you expect from 5 2KW heaters running on 202V and a soldering re-work station of about 160W max. ?


[1] I am assuming that the wiring was done before 17th edition amendment
3 and the 5% reduction in Cmin.

I'm not sure that is relevant to this.