Fuse carriers with fuse fail neons

I believe the circuit resistance for the neon (as opposed to LED) is around 570KOhms


Personally, I wouldn't want to be relying on that particular type of fuse holder to achieve circuit isolation by withdrawing the fuse whilst leaving the neon in place.


In a DB, these is potentially the option of operating the main switch of the DB for isolation purposes - but that does seem a bit brutal in an operational environment.


I'm not entirely clear that if the fuse holder is moved to the position where the fuse can be withdrawn then the neon is also isolated  - but clearly the holder can be returned to the "closed" position even if the fuse is removed and returning the neon to circuit


Regards


OMS

I suspect it is maybe a regs fail, but not actually a problem in practice, and very common in certain types of field kit designed to use "power of opportunity" in which ever country the kit ends up being dropped into.

Although disapproved of, there are such things as neon screwdrivers, where the current to light the lamp flows through the body of the user - the point is it is quite possible to light the smaller sizes of neon indicator without exposing any downstream worker to a dangerous voltage or even a current large enough to cause a sensation they would be able to detect. They do  need a minimum current of a hundred microamps or so to operate, and indeed some load is needed to ensures the collapse of the load -side voltage to a safe value. Even with all loads off, unless the circuit is very short, the wiring capacitance will pass enough to keep the neon on.- a thousand pF (10 metres) is of order 3j megohms at 50Hz, and is enough to strike a dim light. If this  is an  issue then an RC snubber at the exit terminals can be used to provide a minimum load.

I have certainly designed fixed kit with this feature in, but you won't be able to get a true open circuit in meggar terms with the fuse out and to be pukka  it probably needs an additional all-pole isolator, and a warning to use it before opening the box.

I've taken a closer look at these fuse carriers.

The fuse, as you would expect, disconnects from the supply and load terminals as you open the hinged cover.

The neon has contacts to the fuse in the cover that don't make to the supply and load terminals.

So if you leave the fuse out you've got an air gap and good isolation.