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#207524 - 11/03/12 02:32 PM Flickering lights
ANNEMARIE Offline
Member

Registered: 10/11/12
Posts: 41
Loc: DORSET UK
I have a question i have noticed that sometimes when i switch off the fan heater in my workshop the lites dip slighly befor resuming full brightness. I can understand a dip at switch on but not off. I have seen this effect on other large loads in other places as well. Cheers annemarie


Edited by Trumpy (11/05/12 12:52 AM)

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#207529 - 11/03/12 09:20 PM Re: Flickering Lights [Re: ANNEMARIE]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
If it is a single phase center tapped system, I suspect a loose or undersized neutral. The same effect when lights on the other leg brighten when you turn on a heavy load on your leg. The leg "A" to neutral voltage drops because of the load while the neutral to leg "B" voltage increases. The leg "A" to leg "B" voltage remains constant.


Edited by Trumpy (11/05/12 12:53 AM)

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#207532 - 11/04/12 07:12 AM Re: Flickering Lights [Re: LarryC]
ANNEMARIE Offline
Member

Registered: 10/11/12
Posts: 41
Loc: DORSET UK
Unfortunatley thats not it because the supply is just 1 phase of a 3 phase distribution system i only have single phase because the heater is the largest load at 2 kilowatts but thanks for your reply. Annemarie


Edited by Trumpy (11/05/12 12:53 AM)

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#207537 - 11/05/12 12:50 AM Re: Flickering Lights [Re: ANNEMARIE]
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8530
Loc: SI,New Zealand
Hi AnneMarie,
I have a theory on what is happening here.
Us down here in New Zealand, use pretty much the same sort of electrical system that the UK does (with some local variations, of course).

I struck the very same thing you are describing, although it was a while back.

First off, I'm inquisitive to know how this workshop is fed from the "consumer unit", is it fed via a "splitter unit" in the workshop itself or are the socket-outlet and lighting circuits fed individually straight from the consumer unit in the house?

Second question is, how bad is the "dip" when you switch the heater off?, are we talking just a slight dip in lighting level or are we talking "brown-out" territory?

The heater I had the problem with was the same sort of thing as you are describing, it was a 2.4kW fan operated heater, the largest you can supply here on a 10A plug.

What I did at the time, was used what is known here as an interrupted phase tap-on plug, to measure the current drawn by the heater during it running and after it was turned off.
Now you don't need one of these things, if you have a clamp-meter, that will fit into the box behind the socket-outlet that supplies the heater, to test the phase wire current.

What I did find at the time, that at the instant the switch was thrown to turn the heater off, it was like the motor winding was acting like some sort of a choke and creating a "back-EMF" that was upsetting the rest of the electrical system, in the office where it was being used.
Although at the time I wasn't fully convinced that that was the entire cause.

I took the heater home with me, to see if I could replicate the same results, with the 3 phase supply of known quality, I had in my workshop at the time.
Sure enough it did the same thing, although this time I had an oscilloscope on it.
Upon switch off, the current from the heater sagged wildly, into the negative part of the trace, in so much so that I had to change the scale to do the next test.

AnnMarie,
I don't know that I've helped with explaining this at all, something like this is pretty rare and I'd not seen it since, until you raised the question.

If you are still needing answers, by all means, tap out a post, that's what we are here for.

Mike T.
cool


Edited by Trumpy (11/05/12 12:53 AM)
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#207543 - 11/05/12 02:03 PM Re: Flickering Lights [Re: Trumpy]
ANNEMARIE Offline
Member

Registered: 10/11/12
Posts: 41
Loc: DORSET UK
HI Trumpy thanks for your reply my workshop is fed from a 13 amp outdoor socket this then feeds 3 double 13 amp sockets which supply lighting and heating in the little room which is my shop. Incidentley i can stop this effect by turning the heater switch off more slowly rather than just turning it sharply. the dip is just a quick event and i would say its more of a dip than a brownout. I was wondering if it mite be caused by the sudden voltage spike causing the heaters to0 draw more amps momenterily what do you think annemarie

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#207554 - 11/06/12 03:40 AM Re: Flickering Lights [Re: ANNEMARIE]
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8530
Loc: SI,New Zealand
Ann-Marie,
Let me get this straight:
Quote:
my workshop is fed from a 13 amp outdoor socket this then feeds 3 double 13 amp sockets which supply lighting and heating in the little room which is my shop.

So, I am to understand, you are effectively running your workshop as an appliance?, with it fed from a single circuit plugged into an exterior socket-outlet?
That sort of thing is never going to comply with BS 7671 and I hope to God that it has been installed with at least 2.5mm2 wire, because if it is less than the sub-circuit wiring, you are creating a fire hazard.
You need a fused spur for an install like this, permanently wired into a splitter box for out-buildings.
_________________________
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#207562 - 11/06/12 12:35 PM Re: Flickering Lights [Re: ANNEMARIE]
geoff in UK Offline
Member

Registered: 12/30/02
Posts: 169
Loc: UK
The whole lot is protected by the 13amp fuse in the first plug. The socket (assuming reasonably current standards) will be RCD protected. I suspect quite a lot of examples of this type of arrangement could be found in UK.
I also doubt if it would be condemned during an installation inspection, because, as you state, it is effectively an appliance.

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#207563 - 11/06/12 12:56 PM Re: Flickering Lights [Re: geoff in UK]
ANNEMARIE Offline
Member

Registered: 10/11/12
Posts: 41
Loc: DORSET UK
The socket that feeds the workshop is protected by the plug fuse and by a 16 amp trip in the distribution board which also is 100mA rcd. the sockets that are over my repair bench are protected bye a 5 amp fuse and 30mA rcd.I cant change anything because its not my house annemarie

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