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.

 

[Clive P]

Hi,

I think there is something amiss here, as Tony says type C or D's should handle any inrush currents; failing that a conactor ( which you already have ) with each heater protected separately. What size and type is your main protective device?

regards

 

[Knobhead]

The theory is quite simple. Some conductors have higher resistances when hot. For that reason some heaters will draw more current as they are starting up. I would have expected an electrician to know this.

I've ONLY spent 40 years working on kilns and furnaces. Stop faffing about and get you're test set out and do some real testing. There's an underlying fault!

 

[buttonmoon]

I've opened everything apart from looking under the paving stone base and there is no sign of any damage heat. Do you think that there might be a IR fault which only becomes apparent as the materials change shape? If that were the case then it would trip if you turned it on when it's hot which it doesn't. It only trips if all four heaters are plugged in and it's starting from cold. This means that it can't be a fault of an individual heater, the fact that the two MCBs trip together also means it's not the MCBs.

 

[buttonmoon]

I wasn't there when the main trip went but as this is a dedicated circuit and they told me that some other equipment went at the same time I have to assume it was the main switch. which was a 100A double pole main switch on a Proteus board. The reason it tripped is that with the additional current of 4 x 57A, albeit temporarily, added to the rest of the load, on a busy Friday night, it had been tripping the 32B breakers, the 32Cs that I installed allowed the current but the main switch didn't.

 

[buttonmoon]

Consort Claudgen Softstart Control for Quartz Heating

This QSC1 is specifically designed to control the operation of up to 6 kW of Quartz Heaters. It provides a soft start when initially switching the heaters on, thereby removing the surge normally associated with this type of heater. Thanks for the link tlangdon12

 

[ipf]

How does a main switch 'trip' ??

 

[Knobhead]

PS it won't have anything to do with inrush. Did it work before you started playing with it?

 

[Engineer54]

Is the main switch as you call it a 100A MCB/MCCB?? I ask because most main switches in CU's are just that a switch (isolator) with no incorporated protective components....

 

[Knobhead]

How does a main switch 'trip' ??

Good point mg_smile:

 

[telectrix]

or is it an RCD?

 

[Knobhead]

What wories me is he's probably going to chage some poor Schmuck for his professional time.

 

[monkeyelectric]

How does a main switch 'trip' ??

Hey! I asked that earlier, copycat

 

[Engineer54]

or is it an RCD?

Now that entirely possible, and if such, the fault on this unit will be due to an earth leakage problem, and not overcurrent!!

 

[MDJ]

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,

 

[MDJ]

if it is an RCD tripping it would have been nice to know 28 posts ago