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Topic Title: Unexpected dimming of lights.
Topic Summary: Why is it happening when the load is removed?
Created On: 22 September 2017 09:43 AM
Status: Read Only
Linear : Threading : Single : Branch
 Unexpected dimming of lights.   - potential - 22 September 2017 09:43 AM  
 Unexpected dimming of lights.   - kellyselectric - 22 September 2017 12:14 PM  
 Unexpected dimming of lights.   - nad - 22 September 2017 01:00 PM  
 Unexpected dimming of lights.   - mapj1 - 23 September 2017 12:15 AM  
 Unexpected dimming of lights.   - potential - 23 September 2017 10:16 AM  
 Unexpected dimming of lights.   - mapj1 - 23 September 2017 12:32 PM  
 Unexpected dimming of lights.   - OlympusMons - 24 September 2017 02:12 PM  
 Unexpected dimming of lights.   - potential - 24 September 2017 05:41 PM  
 Unexpected dimming of lights.   - potential - 24 September 2017 05:44 PM  
 Unexpected dimming of lights.   - MWalker86 - 24 September 2017 11:10 PM  
 Unexpected dimming of lights.   - kellyselectric - 26 September 2017 12:48 PM  
 Unexpected dimming of lights.   - kellyselectric - 05 October 2017 11:12 AM  
 Unexpected dimming of lights.   - alancapon - 24 September 2017 06:11 PM  
 Unexpected dimming of lights.   - kellyselectric - 24 September 2017 07:52 PM  
 Unexpected dimming of lights.   - potential - 05 October 2017 01:57 PM  
 Unexpected dimming of lights.   - potential - 05 October 2017 02:01 PM  
 Unexpected dimming of lights.   - kellyselectric - 05 October 2017 04:41 PM  
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 24 September 2017 02:12 PM
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OlympusMons

Posts: 50
Joined: 05 February 2016

The shed might be TTd, and not connected to the house earthing, so it might have something to do with earthing.
I am thinking the main house is TNC-S with a broken PEN, so the neutral current is flowing down the bonding, heating up the earthing conductor and raising the voltage on it. When the blower is switched off, the cable cools, the voltage changes and it is this which causes the lights to dim. I am probably completely wrong though.
What does the data plate on the blower say?
 24 September 2017 05:41 PM
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potential

Posts: 1632
Joined: 01 February 2007

The blower is marked 230 volts, 50 hz, 2.2 kW.
The house earth is exported to the shed via the 3rd core.
The bonding in the house is 10mm and very short, under a metre from the MET to the lead water pipe and gas meter supplied via a steel gas pipe.
The supply cable is very old and relatively small.
There is no connection within the house between the MET and the incoming neutral.
The MET has a 10mm wire running to the supply cable steel sheath.

Using a clamp meter there was less than 6 mA flowing between the sheath and MET.
The load in the house was small (as was the load when the phenomenon occurred) being only 1.65 amps.


I did a few more tests but my wife is out this afternoon so wasn't able to repeat the original scenario.
I did not see any dimming of lights in the house except on switch on. There was no dimming after switch off.
I couldn't check in the shed.
I did run an extension lead from the house socket to the shed but the lights did not dim when the blower was switched off.
(This was as I expected)

I checked what voltage exists between the house neutral and a true earth (screwdriver stuck in the lawn).
It varied as I'd expect, but at the time the voltage was less than 2 volts.
When I applied a 4 kW load via another circuit running directly to the CU this voltage increased to 3.6 volts.
I am aware that the supply voltage to the house varies a great deal, anything between 244 to 207 volts so I will monitor this in relation to the supply voltage in the next few days.
 24 September 2017 05:44 PM
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potential

Posts: 1632
Joined: 01 February 2007

I forgot to mention that it was not easy to see any dimming in the house because of light coming in the windows, unlike in the shed which has no windows.
 24 September 2017 11:10 PM
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MWalker86

Posts: 97
Joined: 05 June 2017

Originally posted by: OlympusMons

The shed might be TTd, and not connected to the house earthing, so it might have something to do with earthing.

I am thinking the main house is TNC-S with a broken PEN, so the neutral current is flowing down the bonding, heating up the earthing conductor and raising the voltage on it. When the blower is switched off, the cable cools, the voltage changes and it is this which causes the lights to dim. I am probably completely wrong though.

What does the data plate on the blower say?


This would be more plausible if it wasn't such an instant and brief effect (as described)
 26 September 2017 12:48 PM
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kellyselectric

Posts: 167
Joined: 22 July 2016

BUMP!
 05 October 2017 11:12 AM
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kellyselectric

Posts: 167
Joined: 22 July 2016

So guys am I right in thinking then that the notable dip in lights when something like a 2Kw heater is switched off or when its own thermostat switches it off is simply due to the inductance of the supply lead creating a momentary voltage that pushes against the supply voltage and causes a brief dip? I have noticed this effect is more pronounced with supplys that are a bit weak for instance in a factory I worked in wehad several large ovens and they never made such an effect because we had a big TX just outside in the yard
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