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Topic Title: Neon Indicator and Operation of 32 Amp. M.C.B.
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Created On: 21 September 2017 09:41 AM
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 21 September 2017 09:41 AM
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Joined: 20 February 2014

Mornin All,
yesterday an old boy called me out as he said that in the night he had operated the 20Amp. D.P. switch that controls his immersion heater. There is no time switch. He was trying to get the cheaper night time heating of his water I imagine. He said that there was a flash of light from the switch and then he lost power. The immersion heater has a switch with a neon indicator. The supply for the immersion heater is taken off the ring final and is protected by a B32 M.C.B.

The M.C.B. had operated, he then reset it.

I tested the immersion heater as being o.k. The system is TT earthed with a 30 m.A. R.C.D. I changed the rod stat for a new safety type with the over temperature limit switch.

Could a burnt out series resistor to a neon indicator cause an 32 Amp M.C.B. to trip off? It was sleeved but the sleeving was heat damaged and blackened.


 21 September 2017 10:06 AM
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I rather doubt that a neon indicator itself could pass the 100 amps plus needed to quickly open a 32 amp MCB.

The wires into the neon assembly could pass that current very briefly, perhaps the insulating sleeving had burnt/perished/carbonised and permitted a short circuit between the phase and neutral wires into the pilot lamp.
The fault current would likely vapourise enough of the wires to the pilot lamp to stop the fault recurring, thereby explaining the MCB holding in afterwards.

Fitting a new pilot lamp unit and examining the old one would reveal such a fault.

Alternatively it might have been poor workmanship where the flex was connected to the immersion heater, a few errant strands could have made a short that was later vapourised.
 21 September 2017 01:18 PM
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In the normal switch with pilot, there is a neon lamp and a series resistor of a hundred K or so. The current in the neon is then between 1 and 2mA, and a glow discharge results with a voltage drop of 60-80V .
Normally when they die either the resistor goes open circuit and the neon fails, or the electrodes of the cheaper lamps end up evaporating and re-depositing on the glass as a silvery black deposit.
It is far more likely that an arc was draw that eventually omitted the neon and the resistor altogether. The thin wires to the neon may have served as a guide for the arc, if the damaged insulation provided a region of reduced clearance.

That said, I have (mis?) used the capsules from neon lamps (available on their own as spares or for those who like to bring their own resistors along) as voltage limiters, to protect electronics with antenna inputs and similar, and have noted there is a rather odd effect as the current increases.
Initially at higher currents (10-20mA) the lamp brightens and the voltage falls, but only slightly, presumably as there is more impact ionisation. In this state the device runs hot, and left for more than ten seconds or so will rupture.

However, if you have much lower impedance supply (at least of short duration), such that many amps will flow, then the discharge changes suddenly from orange to blue, and the voltage drops from 50-60 to about 5-10 V. As such, it still can't stand many watt-seconds before it blows up, but is ideal for slicing spikes off waveforms. The current needs to fall below the mA or so for normal orange discharge to restart.

regards Mike
 22 September 2017 08:09 AM
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Thanks broadgage and Mike,
they are useful observations.



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