Irritating electical prob.

[TonyK]

I have a number of electric storage heaters on a single ring covred by a 30A MCB. The heaters are 3 x 1.7Kw and a single 3.3Kw. These are all on 24hr fused spur timers and set to come on at times that offset the load, i.e. the

3.3 and a single 1.7, then two 1.7's etc etc. So in theory the max load is a 3.3 and 1.7 or 5Kw which according to my understanding is 5000/240 = 20.8333 or 21amps? So... why does it trip out occasionally (once every 24 horus) ? No other loading on the circuit but the MCB buzzes and the buzzing gets louder the higher the load, ie. put another .17kw heater on.

So, is there something fundamentally wrong with my understanding or what?

Thanks

[Set Square]

Are you sure that the on periods never overlap, so as to have them all on at the same time. If you do this, you have 8.4kW = 35 amps.

If that's not the case, it might be a transient current problem. Heaters which draw 20 amps in the steady state probably draw considerably more current than that for the first few seconds when you turn them on - because the element resistance will be quite low when cold. Maybe you need a trip which can stand a short-term overload without tripping.

[Scott]

Almost... It does depend on the type of load. These will be broadly resistive so yes, BUT I'd say it would be a slight inductive load that is causing this problem.

So... why does it trip out occasionally (once

I'm wondering if you need an MCB with a different swiching characteristic - type C or D. Basically you get a slight short circuit on switch on which could be knocking out the MCB, a C/D type will allow for this before tripping.

Now that is odd...

Scott

I would say that upon switch on they

[Lurch]

On Sun, 28 Nov 2004 20:23:18 -0000, "TonyK" strung together this:

Oh dear, a bad start.

Well, 3.3kw is over the maximum size of fixed appliance that should be connected to a ring final circuit. The spur will no doubt be rated at

3kw.

Hmm, not neccesarily. Depends on what your actual supply voltage is, it could be more than 20.8333A. There is also the probability of a start up current that is greater than the running load.

Probably because you're a bit close to the limit of the MCB with it all on. Assuming the voltage is closer 230 the total load with it all on is 29.summat amps, not a good idea. Only option in my opinion is rewire it with radials to each heater and have a central CU\contactor\timeclock controlling them.

I would say so, yes.

[Andy Wade]

Absolutely not. Storage heaters have nichrome elements which do not have any significant switch-on surge. The small amount of inductance in the circuit will not have any significant effect either. A type B breaker is entirely appropriate here. In any case changing an existing type B MCB to a 'C' or especially to a 'D' without checking the earth fault loop impedance etc. could be very dangerous.

As Lurch has said this is a mad way to wire up storage heaters, but that doesn't explain why the MCB is tripping. The small overload when all the heaters are on (~36 A through a 32 A device) should not be enough to cause the MCB to trip. So either the MCB is wrongly marked or faulty (fairly unlikely) or something is drawing more current than it should be. The OP needs to get hold of a clamp ammeter and start measuring currents.

[TonyK]

Thanks for the pointers. I'm pretty well stuck with the way the heaters are installed and this is one of three sets of heaters in the building. The others are fine so I feel the problem is with this one circuit. The timers are programmed with the heaters set to go on and off in groups so (in theory) there is never more than 5Kw draw at any one time however 2 heaters do switch on pretty simultaneously so I'll offset them by a couple of minutes and see. Plus I'll exchange to MCB with another and see if that is faulty in any way.

[usenet]

Heater wire resistance changes very little with temperature, there won't be a significant inrush current.