Heater start up currents

[buttonmoon]

Hi fellas

I've got an outdoor heating circuit that I'm trying to fault find.

There are four heaters under a parasol each rated at 1.5kW that are tripping two MCBs in series. One is on the main board the other is in a separate unit dedicated to the outdoor circuit. When the heaters are cold if all four heaters are plugged in then both Mcbs trip instantly. If only two are plugged in it starts and then you can plug the other two in and there's no problem. Then when it times out if it's switched back on immediately then they come on without a problem.

Both MCBs are B types BSEN 60898 hager and MK.
There is no problem with IR
The measured resistance of a heater when cold is 4 ohms
This would give each one a starting current of 57.5 A
So the four together would have a startup current of 230 A or 52 kW

My first thought was to change the MCBs to C or D types but on reflection of these figures that would just result in a tripped main switch. I am thinking of some sort of soft start current control option but I haven't used one before and I'm not sure if that's the best solution.

Apparently it used to work, but I don't see how.

All thoughts welcome.

 

[timo1]

Am I being really dumb and missing something?...the four heaters pull approximately 26 amps when on together? a C type breaker should handle this and in any case the load should not be touching the 32 mcb. Is it an RCD main switch, maybe there is a fault in one of the heaters? I'd be interested to find out what the answer is on this one.

 

[ipf]

Proteus crap anyway.

 

[Engineer54]

I can't imagine anyone calling a RCD a main switch though, it may be a incommer in a dist board, or connected before a board but surely if it were a RCD then we would have been told by now, a pretty big thing to keep quiet in this case if so haha,

More likely a DP MCB.. lol!!

 

[MDJ]

looks like it may be an RCD main switch, but I would have thought if the case it would have been mentioned and not just main switch tripped, very strange this one.

 

[ipf]

How did the main switch trip?

OK :book:

 

[Knobhead]

Are you old enough to remember Spitting Image? The Presidents Brain is Missing?

 

[Octopus]

Where has the OP gone?

 

[buttonmoon]

Ok We'll start again. The only time this new 'device' tripped is when I wasn't there after I changed the MCBs which were tripping two C types. There is an RCD on the dedicated board which hasn't tripped at any point. Fair point about the main switch thing but it's academic really because if it was an earth fault the other RCD should have tripped before any of the MCBs.

 

[Octopus]

What actual testing have you done?

 

[buttonmoon]

I tested the continuity and IR of the individual heaters and the whole circuit and I did Zs at the contactor. My clamp meter was out of batteries so I couldn't do a live current test.

IR was off the scale, Zs was normal as was continuity of the circuit. The only reading I thought was add was 4ohms resistance on the appliances.

 

[Knobhead]

Now that's real testing:
"Off scale"
"Normal"
"Batteries flat"

 

[buttonmoon]

OK off the scale=>500 Mohms, normal Zs means definately less than 1.44ohms as per p49 BS7671 Max Zs for Type B 32A bsen60898 MCB but in fact now that I have my test results with me 0.42ohms and batteries flat well I have no excuse for that one, seeing as how you're so keen to judge I'm sure you've never run out of batteries on a job.

However all that said, from your own 40 years of experience rather than criticising my terminology perhaps you could suggest a solution other than the one you've already suggested which I had already tried and which failed.mg_smile:

 

[Octopus]

OK off the scale=>500 Mohms, normal Zs means definately less than 1.44ohms as per p49 BS7671 Max Zs for Type B 32A bsen60898 MCB but in fact now that I have my test results with me 0.42ohms and batteries flat well I have no excuse for that one, seeing as how you're so keen to judge I'm sure you've never run out of batteries on a job.

However all that said, from your own 40 years of experience rather than criticising my terminology perhaps you could suggest a solution other than the one you've already suggested which I had already tried and which failed.mg_smile:

Is it me or is this forum getting infested with people who can't take critisium??

 

[buttonmoon]

Lol I'm ok with the criticism, what I'm after is a solution. I've had three pages of responses and only 1 suggestion that I haven't tried.

If heaters can't have high start up currents why are there companies that make products which control those currents?

If its not a high start up current why do they all turn on ok when they're already hot?

If I've got it wrong... well isn't that what this forum is for?

Oh and unlike some, I never charge if I don't fix it and I like to be sure which is why I on this site.

But the criticism is fine I'd just like to get this sorted so the people outside the pub can stop freezing their arses off.

 

[Octopus]

If you want to make it dummy proof you need to install 2 more circuits!