Fusing a pilot light

[Krolroger]

Good evening,

Can anyone tell me whether it is good practice to fuse a 240VAC pilot/standby light which in my case is RS stock number 207-113 ?

I'm wary of sticking anything across the mains without a suitable fuse but wondered whether this device is likely to contain a fusible resistor which would presumably render an external in-line fuse redundant?

There's nothing in the datasheet.

Thanks,

Simon

[G6Tanuki]

To me this is truly over-analysing things. I've never seen a situation where a competent designer has included a fuse in a pilot-light circuit.

The bulb is probably faster-reacting than any fuse - indeed I'm familiar with the use of a 6V 250mA MES pilot-light bulb in the centre-tap-to-earth of a traditional valve-amplifier HT supply where it serves both as a pilot-lamp and a fuse !

[winston_1]

Pilpt lights are not normally separately fused. As it takes only 3mA you would be pushed to find any relevant fuse.

[Krolroger]

Thanks for that.

In fact it's an LED with a 3mA current draw at 240VAC.

Simon

[Krolroger]

Thank you both.

Off topic I know, but would you apply the same principle (current draw too small to fuse effectively) to a 240VAC relay coil with a rated coil current of around 5mA?

[julie_m]

There is almost certainly going to be a series resistor inside the indicator; and nowadays this is almost certain to be a metal film type. Even if not specifically sold as a fusible resistor, such a resistor will always fail open-circuit anyway under fault conditions -- what's special about a proper fusible resistor is, it will never set fire to anything else. The kind of fault that would cause a generic resistor to start a fire is rare anyway (it would require the current to increase just a bit above normal, so the resistor cooks slowly and has time to pre-heat surrounding parts; usually something fails dead short, the resistor gets the full force of the mains and fails instantly).

The power indicator is usually wired downstream of the appliance's fuse, and in any case there will be a fuse in the plug.

Given the fairly remote likelihood of a fault even occurring, let alone the minimal risk of complications, it's probably not worth worrying about..

[Diabolical Artificer]

An interesting one this, I recently had an HH amp on the bench with a cheap and cheerful rocker SW with internal neon indicator inside that had blown with the negative wiring cut near the SW and a bigger fuse in the fuse holder.

No idea what the original fault was or why the wiring was cut, but whilst fixing this issue i replaced the SW only to have the neon blow in another cheap arsed SW resulting in the lead fuse blowing. This was weird as I had a 1A instead of 2A in the amp for testing.

Since then I've read on another forum of HH amp rocker SW's causing issues, so do neon indicators go SC when they fail? Not saying they need a separate fuse but the OP's question got me thinking.

Andy.

['LIVEWIRE?']

Andy, I have recently repaired a HH VX-150 in which the Rocker switch had failed (one pole o/c). This led to the Bridge rectifier going almost completely s/c, which took out the two 5A(T) 20mm Fuses in the Mains Tx secondary. In that case, once the aforesaid components had been replaced, the amp. was back in full working order, other than the fact that the slide switches on the input/preamp. board were crackly & intermittent, which I put down to age and repeated use. Maybe it was my thread on diyaudio.com you saw. As you know those red rocker switches with in-built neons are commonplace things. I have one in a 13A socket strip on my computer desk, and at least one other in a socket strip in my workshop, neither of which, so far, have caused any problems, but have two new ones in my spares box in case of need!

[Biggles]

Could the neon that failed been exposed to mechanical shock caused by shifting and loading/unloading the amp while attending gigs? Neons normally have two closely mounted electrodes which depending on the construction quality could become too close or even shorted out if subject to a hard knock. Not something I have come across before though.
Just an idea.
Alan.

[McMurdo]

If the neon goes short nothing will happen thanks to the series resistor of course, without it, even a good one will pop somewhat. I've had neon rockers go short but it's usually been due to the internal resistor causing carbonisation inside the switch and then tracking across.

A particular product we used to service had mains-operated LED-resistor combinations and the resistor heat used to carbonise the heatshrink sleeving and put the full mains across the LED, with fuse-blowing results.

But neon indicators have been wired across the mains for decades with no ill effect, I'd go right ahead. What's the worst that can happen?

[Diabolical Artificer]

"Maybe it was my thread on diyaudio.com" Could be, the neon in mine had well blown, the glass being blackened but maybe that's normal.

Thanks Kevin, that's good to know.

A.