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,

W/V=I 1500/230= 6.25A

P=I Sqd x R Therefore R=P/I Sqd 1500/42.5=35 Ohms.

4 Ohms seems a little low, what size MCB's are protecting?

Regards.

 

[buttonmoon]

Oh sorry, both are B32. Running current according to rating should be 1500/230=6.52A

 

[buttonmoon]

Total of 6.52 x 4 = 26.08A which explains why they can run together but can't start together.

 

[Clive P]

Hi, I stand corrected 6.52A

Regards.

 

[Clive P]

Hi,

Possibly split the circuits or wire through a contactor with seperate protection to give greater flexibility,

Regards.

 

[tlangdon12]

Might one of the soft start devices on this page be suitable? http://aquahot.co.uk/index.php?action=store&ctg=42&show_ctg=138

 

[Knobhead]

Put a type C or D in.

 

[buttonmoon]

Right well this is the next thing. The current for the heaters is switched through a contactor. As I said before, I haven't used one of these soft start units before, but from what I can see they appear to work through the switch. I assume they work by having a high resistance added to the circuit which would decrease either though a timer or some temperature control thereby reducing the start up current but not interfering with normal operation. If that is the case then having on at the switch in this case wouldn't work because that would only affect the switching current and not the current which goes through the devices when the contactor switches on the load current.

Is there a soft start contactor or should I just recommend that they change their heaters, rewiring their circuit would be the most expensive option.

Oh if I put a Cs or Ds in it will blow the main switch

 

[Knobhead]

Soft sart for heaters? Soft start contactor?

I'm begining to loose the will to live.


Which planet are you on?
Get an electrician in.

 

[buttonmoon]

"Put a type C or D in."

I tried your idea last night Tony and the main switch went.

 

[tlangdon12]

The soft-start unit would be installed between the contactor and the load.

 

[buttonmoon]

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.

 

[dim_bulb]

if you can take the cover off perhaps its chared the cables inside
and do an IR test a soon as it trips perhaps when it starts to get warm

 

[monkeyelectric]

How did the main switch trip?