[mburtis]

You do this all the time on trailer lights, that's 12v though. Im still trying to wrap my brain theoretically what problems this could actually cause.

 

[joe-nwt]

You do this all the time on trailer lights, that's 12v though. Im still trying to wrap my brain theoretically what problems this could actually cause.

It will cause massive currents between where the light is grounded on the door and the ground on the control transformer. Could cause anything from Ferroresonance to a temporal rift in the space/time continuum. Not to be taken lightly...

 

[paulengr]

I have a question for you: have you ever seen an mcc bucket/starter that also contained a circuit that put about 12VDC on the ac motor when it is not running?
My off shore drilling rig did. I asked and was told it was to keep the motor warm and avoid condensation when not running. Most of our motors were explosion proof.

Off topic but yes. It’s a built in function in a lot of drives.

 

[paulengr]

What the OP mentions is actually mandatory in NEC in some cases or a variation of it. In ungrounded systems phase lights or some similar ground faults detection is mandatory. In this system there are THREE lights, not just one. PTs (potential transformer) are often needed and legal. All of this assumes things are over 240 V as per Code limitations on ungrounded systems. The PT itself is a huge impedance and limits current to a few mA at best.

With a full set of 3 phase lights normally all 3 lights have the same brightness. When a ground faults occurs one of the lights goes out or gets dim while the other 2 get much brighter. This is because on say a 480 V system normally the voltage to ground is approximately 277 but when a ground fault occurs the faulted phase is 0 and the undaunted phases are 480. I’ve never seen a single phase light but there shouldn’t be anything inherently wrong with it on single phase loads.

 

[mburtis]

Do I got this right? Under normal operations there would be a small amount of current flowing through the panel between the light ground and the transformer ground, limited by the resistance of the light and at ground potential. If you lost the x2 ground at the transformer the polarity basically switches and x1 becomes the grounded lead and x2 would suddenly become the hot lead.

 

[splatz]

Do I got this right? Under normal operations there would be a small amount of current flowing through the panel between the light ground and the transformer ground, limited by the resistance of the light and at ground potential. If you lost the x2 ground at the transformer the polarity basically switches and x1 becomes the grounded lead and x2 would suddenly become the hot lead.

I thought it was simpler than that. If you power the lamp with x1 on the hot terminal and a ground jumper for the neutral side of the lamp, the neutral side path is through the metal to the ground jumper to the transformer X2. The current is limited to the current the lamp draws. If the x2 loses it's ground, the lamp loses it's neutral side path.

Thinking about this, I'd be less skeeved by a green wire from the neutral side of the lamp back to the ground lug on the door. Just the idea that the door isn't normally a conductor. Plus you'd get an indication when the door loses it's safety ground.

My original idea was to use a relay near the transformer with a green wire to a ground and the hot to the lamp through the NO contacts of the relay. Now I am thinking that ground wire on the neutral side of the coil could go to that door ground lug. The relay could also power a two color lamp, the NO contacts the green light and the NC contacts the red light. If you wanted to you could even interlock the machine with that relay, depending on the machine.

 

[joe-nwt]

The only reason for that light to even exist is there must be other devices on the machine that depend on a ground connection to operate. Some type of sensor most likely.

 

[SteveBayshore]

On the old machines that I have run across, the function was to indicate that the EGC with power feed to the machine was in tact. Kind of a cheap way of having a ground indicator, not really the integrity of the neutral connection on the transformer.

 

[joe-nwt]

On the old machines that I have run across, the function was to indicate that the EGC with power feed to the machine was in tact. Kind of a cheap way of having a ground indicator, not really the integrity of the neutral connection on the transformer.

That light only has reference to the ground to the X2 of the control transformer. It has no reference to the ECG from the feed.

 

[Dennis Alwon]

If there is a trany why isn't the light tied to both sides of the trany- I assume a 240V to 120v trany. I agree a 12 or 24v light from a trany would be the way to go

 

[cuba_pete]

Also didnt think the nec covered equipment panels.

I didn’t want to engage, but someone else did chime in:

What the OP mentions is actually mandatory in NEC in some cases or a variation of it. In ungrounded systems phase lights or some similar ground faults detection is mandatory.

Oh, really? Thanks!

I haven’t bothered to try to find the section…maybe tomorrow…my kid is getting married today.

 

[paulengr]

Do I got this right? Under normal operations there would be a small amount of current flowing through the panel between the light ground and the transformer ground, limited by the resistance of the light and at ground potential. If you lost the x2 ground at the transformer the polarity basically switches and x1 becomes the grounded lead and x2 would suddenly become the hot lead.

You are too hung up on a “ground” in an ungrounded system. Actually ignoring impedance X1 and X2 are at the same potential. In reslity impedance exists. The potential at the fault is zero almost by definition. As you increase distance from the fault towards the power source L1 it picks up capacitive and inductive currents to L2 and L3 and the impedance path from X1 and L1 may be very low but in any cases there will be a higher voltage as the distance from the fault increases. This is the basis behind equipotential safety grounds or “work site” grounds as opposed to the former “bracket ground” concept. Keep in mind though that this is basically “stray” voltage between X1 and X2, only detectable if the pilot lamp and transformer impedance are much higher than the distributed capacitance and inductance involved or effectively the phase light just grounds it out,

 

[paulengr]

I didn’t want to engage, but someone else did chime in:


Oh, really? Thanks!

I haven’t bothered to try to find the section…maybe tomorrow…my kid is getting married today.

250.21(A) and (B). This is simple logic. You don’t want to ground delta PTs and in general if you are trying to measure voltage to neutral/ground in a wye circuit grounding defeats the purpose. The phase light is a control circuit.

 

[cuba_pete]

250.21(A) and (B). This is simple logic. You don’t want to ground delta PTs and in general if you are trying to measure voltage to neutral/ground in a wye circuit grounding defeats the purpose. The phase light is a control circuit.

right on…this is where I was going in post #2.

 

[Darr]

Also didnt think the nec covered equipment panels.

Me too. Did you look in NFPA 79 instead?

 

[dontreadthis]

That wiring actually seems really dangerous to me. Part of the diagram is cut off in the photo but I don’t see any fuse on 69. That grounded X2 isn’t doing much good if X1 isn’t fused. It gives the potential for a huge amount of fault current and arc flash. Assuming X1 is a lead coming out of the xfmr, I’d run it straight to a fuse holder and then feed everything else from there.

 

[Djea3]

that feels backwards!
set up a relay to light a warning light when the ground is interrupted and even interlock the machine if it can be stopped safely.

I thought about that as I was reading. I have worked around a lot of this equipment in machine shops. From an operator point of view it becomes "normal", even though the light being "OFF or dim" is the indicator, clearly not a rational plan.

I agree, better to set it through relays that light the RED light, rather than just lighting a green light.
However in over 20 years in manufacturing, I never saw a "ground" fail on equipment, possibly as all was EMT or metal flex or so much mass of metal to concrete with rebar in it that the equipment was self-grounding. I saw relays fail and indicator bulbs fail regularly. Damn those little buggers cost a lot in down time and replacement cost too.

Remember, most was equipment is 220 or 3 phase (always both in the same facility). Ground isn't a big deal, having a phase connect to conductive material that is ungrounded is the problem.

I don't remember any equipment that was insulated under its legs or contact points unless those parts were hard bolted to the rest of the machine with an uninsulated base (like a carriage table on a CNC punch press).

Possibly an entire different way of looking at it? I guess the worst problem would be drop cords to smaller equipment.....no EMT needed, ground could be an issue then. Then again the smaller heavy equipment was pneumatic with 120V and later some battery powered mobile stuff.

If you ask me, the forklift charging stations were just dangerous, you could see the damage to nearby metals from acid in the air. Ovens...autoclaves...uggghh.

We had a 30ft or longer autoclave that went to 400+ degrees, nitrogen filled to HIGH pressure for composites (Stealth fighter/Stealth Bomber/ Satellites and Nuclear missiles), while pulling a vacuum on parts. I always LEFT THE AREA when it was pressurizing. I had pictures of a door lug failure on my desk (same size unit and same door). It took out all the offices and equipment all the way through a tilt up concrete building, blew out the back wall and the door took out something like 30 cars in the parking lot. That I consider dangerous equipment.
My bosses never said a word after the first time I showed them the pictures and and told them that I would be back when the pressure was at max.
Remember that anything an EMPLOYEE DEEMS UNSAFE, IS GENERALLY....UNSAFE. Me sitting in line of the door at pressurization....unsafe.

 

[cuba_pete]

That wiring actually seems really dangerous to me. Part of the diagram is cut off in the photo but I don’t see any fuse on 69. That grounded X2 isn’t doing much good if X1 isn’t fused. It gives the potential for a huge amount of fault current and arc flash. Assuming X1 is a lead coming out of the xfmr, I’d run it straight to a fuse holder and then feed everything else from there.

You’d fuse a lead going to a lamp…a fuse just for a lamp?

 

[dontreadthis]

You’d fuse a lead going to a lamp…a fuse just for a lamp?

Did you look at the wiring diagram? 69 is run to many more devices than just a lamp. Having said that, it doesn’t matter what it’s running to as far as reducing the potential for fault current. If an unfused and ungrounded wire grounds out, there will be a big boom. Primary is 480v, secondary is 120v. 480/120=4. So a ground fault on the secondary will take 4x the fault current of a primary ground fault to trip the primary ocpd. Tbh, the fault would more likely be cleared by the wire blowing itself apart.

 

[cuba_pete]

Did you look at the wiring diagram? 69 is run to many more devices than just a lamp. Having said that, it doesn’t matter what it’s running to as far as reducing the potential for fault current. If an unfused and ungrounded wire grounds out, there will be a big boom. Primary is 480v, secondary is 120v. 480/120=4. So a ground fault on the secondary will take 4x the fault current of a primary ground fault to trip the primary ocpd. Tbh, the fault would more likely be cleared by the wire blowing itself apart.

I did look at the diagram, but I’m not gonna go on some wild goose chase supposing that it doesn’t have fault protection since it is an incomplete diagram, as you yourself pointed out, and further assume that the manufacturer did something wrong when the OP never indicated as such. Seems like you’re going off on a limb here trying to find a problem where one doesn’t necessarily exist. That information has not been given.

We don’t even know if this is sheet 1 out of 100, or more, or less.

It seems like a lot of people are forgetting here that we’re generally not allowed to modify a manufacturer‘s piece of equipment either. If something‘s wrong with equipment design, contact the manufacturer.