On 17/11/2017 12:59, whisky-dave wrote:
On Friday, 17 November 2017 11:05:38 UTC, Andy Burns wrote:
whisky-dave wrote:

what makes yuo think it;s 40 amps will you answer this or not.

I do not think your heaters are taking 40A. We were discussing why
the MCB didn't trip, because a 32A breaker can supply more than 40A
for hours without tripping.

only 140 amps where or how did you get that figure ?

You just glibly replied to Johns message where he showed you ...
he wasn't saying your heaters are taking 140A, rather that in the
even of a short circuit (where ideally you'd want many hundreds of
amps available to ensure the MCB trips in under half a second) your
202V supply could only supply 140A which would take it 20 seconds
to trip.

What makes you think it could only supply 140A ?. Isn't that the case
with all 32A MCB anywhere in the country so if that is a fault of the
MCBs maybe they need redesigning.

This has nothing to do with the MCBs or the heaters - this is asking the
question what happens when there is an actual fault. Under fault
conditions its loop impedance that matters. That will be made up of your
supply impedance, your external earth impedance, and the total
impedance of all the submain and circuit cables between your supply head
end and the location of the fault, added together.

Suppose we were drawing 32 amps (4
heaters) what would be the calculations then ?

Not relevant

140A at 202V is 22kW heating up your cabling, your lab should be
toasty until the fire service arrive.

What makes you think that

but it wouldn't be 140A at 202V would it. If we can only get 40A at
202V

That is through the parallel load of the heaters. Not through a direct
short.

what makes yuo think we could get 140A at 202V surely as the
current increased the voltage between L-N would drop. It droped from
~215V at 0A to ~202V at 40A, I'm pretty sure that once it got to 50
Amps the voltage would drop below 200V.

Which is what makes the situation more dangerous. Should your circuit
experience a fault, it may not be able to pass adequate current to trip
the MCB quickly. Which is what we have been trying to explain all along!

Unless you have a clear understanding of the differences between
overload currents and fault currents, not much of this will make sense
to you.

--
Cheers,

John.

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