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

In an earlier contribution to this discussion,
TonyK wrote:

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

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.
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.

"Set Square" wrote in message
...
In an earlier contribution to this discussion,
TonyK wrote:

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?

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.

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.

Now that is odd...

Scott




I would say that upon switch on they

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

I have a number of electric storage heaters on a single ring covred by a 30A
MCB.

Oh dear, a bad start.

The heaters are 3 x 1.7Kw and a single 3.3Kw. These are all on 24hr
fused spur timers

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.

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?

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.

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.

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.

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

I would say so, yes.
--

SJW
A.C.S. Ltd
Please reply to group or use 'usenet' in email subject

Scott wrote:

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.

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.

--
Andy

"TonyK" wrote in message
...
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



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.

Set Square wrote:
In an earlier contribution to this discussion,
TonyK wrote:

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

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.

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

--
Chris Green