We have test bays at work that use 50A terminal blocks like the one pictured below for quick hookup and disconnect of machines to be tested (208-240VAC, split phase and three phase, up to 40A loads.)

Each terminal block is mounted in a box with a hinged cover and a door-switch such that when the door is open, the switch opens, and that breaks the coil circuit for 50A contactors (in a separate box) which disconnect power from the terminal block.

So, barring any equipment failures, any time the door is open, there's no voltage at the terminals and it's safe to work. Of course, in reality the door switch or the contactor could fail, so the original design also included a neon light across the two hot lines so that you could quickly see whether or not power in the box was really off.

Unfortunately, the neon indicators faded really fast and are essentially invisible now. On top of that, the existing system wouldn't warn you if one contact was welded shut, but the other was open. You could potentially have lethal voltage on line 1, but if line 2 was off, the indicator light would stay dark.

We're revisiting this system now, and I thought maybe we should use two lights per box, one from line 1 to ground, and the other from line 2 to ground. This way if either line is hot, you get a warning light. But I know you're not generally supposed to use the protective earth ground to carry any current. Do you think the current for a small neon (or mains rated LED) light would be ok, or is this a zero tolerance, no exceptions rule? Unfortunately there aren't neutrals in many of the boxes, so I can't use neutrals for the return path.

Alternately, any other ideas for how to provide a more reliable warning/indicator system? Ideally, workers should check the terminals with a meter every time they open a box, but I'm getting no traction on forcing that issue. They want/need some type of warning/indicator that doesn't require the user to grab extra tools.

I'd appreciate any thoughts on both the ground return path and the general warning indicator questions. Thanks!

 

You haven't explained why the original neons have "faded "??? ...neons don't usually fade , can't they be replaced ....

This is only 240V ....I've had dozens of mains shocks ... it keeps you on your toes , as long as you haven't got your feet in a pool of water it's not a problem ....

To be extra safe side why not put a rubber mat under the area where the terminal block is located .

 

MOD Note:
A 240Vac shock can kill you or some other person, ALWAYS put safety first, when working with electrical equipment.

 

A neon indicator bulb needs only a very little current to glow, so it would have not be really a problem to flow to the ground.
If you're in doubt, think on the srewdrivers used to test voltage presence. the current flowing through the neon indicator flows thru the body of the tester man whitout noticing it.

Many electronic equipement have filter condensers from the main supply terminals to the ground. They usually drive more current than a neon bulb.

 

 

I would be wary about any lamp indicator that is intended to indicate a fault condition. What happens when the lamp is malfunctioning as in your case? Safety systems have two lamps, one for ON and the second for OFF. One light must always be on.

For safe lockout, install another circuit breaker on the input side of your box.

 

There is also a 3ph NFB (non-fused-breaker) that has both (or either) a trip coil used when a enclosure door is opened operated via a micro sw etc, and a indicating contact that can be used to show the status of the NFB.
AKA Shunt trip breaker.
Max.

 

A neon indicator bulb needs only a very little current to glow, so it would have not be really a problem to flow to the ground.
If you're in doubt, think on the srewdrivers used to test voltage presence. the current flowing through the neon indicator flows thru the body of the tester man whitout noticing it.

Many electronic equipement have filter condensers from the main supply terminals to the ground. They usually drive more current than a neon bulb.

That's what I thought too, but then I started to doubt myself. Thanks!

 

The neons should last a very long time.
Sounds like they were over driven. They typically only need a mA or so, so select a series resistor to give that current.
The neon operating voltage is about 80V, thus for 240V the series resistor would be (240-80)/1mA = 160kΩ.

 

I would be wary about any lamp indicator that is intended to indicate a fault condition. What happens when the lamp is malfunctioning as in your case? Safety systems have two lamps, one for ON and the second for OFF. One light must always be on.

For safe lockout, install another circuit breaker on the input side of your box.

I agree completely. I remember that when they first showed me this system, I thought it seemed pretty cool briefly, but I very quickly realized there were multiple failure modes that could be misleading. I kind of feel like it's better to have no indicator system at all than to have one that could be wrong and give you a false sense of security.

I like the idea of a two lamp on/off system that leaves no ambiguity.

Sadly, I'm sure my boss won't go for installing extra breakers before every box. He already thinks I'm being too paranoid.

 

The neons should last a very long time.
Sounds like they were over driven. They typically only need a mA or so, so select a series resistor to give that current.
The neon operating voltage is about 80V, thus for 240V the series resistor would be (240-80)/1mA = 160kΩ.

Interesting - I've never used neon lamps and knew nothing about them. I believe these are panel indicators that must have their resistors built-in, cause I'm almost positive that there are no discrete resistors anywhere in the box.

That does make me wonder though - we've got some unusual wiring where we've got both "normal" US split phase and high leg delta three phase in the building (and the test bays are evenly divided between the two systems,) so between the various boxes you can easily find 120, 208, or 240V. It wouldn't surprise me if we had indicators chosen for the wrong voltage. I wonder if that alone would be enough of a difference to matter.

 

It wouldn't surprise me if we had indicators chosen for the wrong voltage. I wonder if that alone would be enough of a difference to matter.

Certainly if the indicator was designed for 120V, the bulb could be significantly overdriven at 240V.

 

Interesting - I've never used neon lamps and knew nothing about them. I believe these are panel indicators that must have their resistors built-in, cause I'm almost positive that there are no discrete resistors anywhere in the box.
.

You need around 2megs/100v you can get panel mount types for appliances etc, you could always add an external resistor for higher voltages.
When neons start to fail, they usually start to flicker.
Max.

 

When neons start to fail, they usually start to flicker.

If the neon current is excessive it may dim from blackening of bulb due to erosion of the electrodes.

 

MOD Note:
A 240Vac shock can kill you or some other person, ALWAYS put safety first, when working with electrical equipment.

To clarify the moderator statement, a 240 volt shock can leave you EXTREMELY DEAD!!! and with a 50 amp circuit it can also evaporate a lot of copper, which creates a very nasty burn.

 

Neon indicators always need to have an adequate current limiting resistor, otherwise they will fail within minutes, or less.
There does exist a safe procedure for using incandescent light bulbs to verify that a circuit is indeed off. First, each line needs to have two bulbs, for redundancy, in case one fails while checking. Then the circuit is switched on to verify that the bulbs are working, and then switched off, to verify that now the power is known to be off.
But my choice is a blade type of disconnect switch where I can directly see that the circuit is physically opened. That arrangement has proven to be 100% effective when it is used. Yes, it does require opening an enclosure with a hazardous voltage, so one must remember to never grab live wires. Probably putting a clear window in the enclosure door would be good.