117V neutral to Gnd ?

S

Shuntme

Member
Location
San Diego California
Occupation
Instrument Control Tech
Lost 120Vac (15A brK Electrical panel) for my IOC cabinet #1. 24Vdc UPS. 120Vac power in through a LAS(surge suppressor) 15A(internal fuse). Lost the IOC as the UPS drained out(2hrs), 15A breaker(panel) did not trip. Had 120V in for the LAS unit but no 120V out. Quick fix was pigtailed a 120V to my 24Vdc power supply to power the IOC get plc and scada back up and running for Operations.
Troubleshooting Vac power: LAS unit was mounted upside down in the cabinet and tight to the bottom. Did not see the pop out 15A internal fuse until unit was removed - Fuse was blown. Tested in/out terminals and no indications of any shorts. With the LAS unit out and 15A panel breaker off I tested hot to neutral and got 67V. Hot to ground 48V and neutral to ground 117V.
Used a LoZ meter the 67V and 48V went away but the 117V neutral to ground was still there. Turned breaker on, 117V hot /neutral, but 117V neutral/Gnd still present.
Reinstalled LAS unit with new fuse(15A) but isolated the plc 24Vdc controls section so it's still running off the pigtail(120V) to DC power supply only.
Smoked the LAS unit immediately.
Weird scenarios:
1. The LAS unit smoked but internal 15A fuse(LAS) for IOC cabinet #1 did not blow and the 15A panel breaker did not trip.
2. My other IOC cabinet #2(exact duplicate but 30ft away) lost its 120V power and went on UPS power for 1hr. I caught it before it shut the operation down. This IOC is on it's own separate 15A breaker but on the same leg in the electrical panel. Replaced the LAS unit(spare) and 120V power was restored right away. However checking this LAS unit the 15A internal fuse was blown and the 15A panel breaker did not trip.
- Do I have a broken neutral in IOC #1 ?
- Why did the 15A LAS internal fuse in IOC #1 did not blow and the panel breaker did not trip ?
- Why did the fault in IOC #1 cause the LAS fuse in IOC to blow ?
 
Electricians work is so complex and varied. A lot of your devices are not familiar but someone may be. But I can say that fuses blow quicker than breakers trip. Usually. Good luck brother.
 
Shuntme said:
Lost 120Vac (15A brK Electrical panel) for my IOC cabinet #1. 24Vdc UPS. 120Vac power in through a LAS(surge suppressor) 15A(internal fuse). Lost the IOC as the UPS drained out(2hrs), 15A breaker(panel) did not trip. Had 120V in for the LAS unit but no 120V out. Quick fix was pigtailed a 120V to my 24Vdc power supply to power the IOC get plc and scada back up and running for Operations.
Troubleshooting Vac power: LAS unit was mounted upside down in the cabinet and tight to the bottom. Did not see the pop out 15A internal fuse until unit was removed - Fuse was blown. Tested in/out terminals and no indications of any shorts. With the LAS unit out and 15A panel breaker off I tested hot to neutral and got 67V. Hot to ground 48V and neutral to ground 117V.
Used a LoZ meter the 67V and 48V went away but the 117V neutral to ground was still there. Turned breaker on, 117V hot /neutral, but 117V neutral/Gnd still present.
Reinstalled LAS unit with new fuse(15A) but isolated the plc 24Vdc controls section so it's still running off the pigtail(120V) to DC power supply only.
Smoked the LAS unit immediately.
Weird scenarios:
1. The LAS unit smoked but internal 15A fuse(LAS) for IOC cabinet #1 did not blow and the 15A panel breaker did not trip.
2. My other IOC cabinet #2(exact duplicate but 30ft away) lost its 120V power and went on UPS power for 1hr. I caught it before it shut the operation down. This IOC is on it's own separate 15A breaker but on the same leg in the electrical panel. Replaced the LAS unit(spare) and 120V power was restored right away. However checking this LAS unit the 15A internal fuse was blown and the 15A panel breaker did not trip.
- Do I have a broken neutral in IOC #1 ?
- Why did the 15A LAS internal fuse in IOC #1 did not blow and the panel breaker did not trip ?
- Why did the fault in IOC #1 cause the LAS fuse in IOC to blow ?
Click to expand...
Im not familiar with your terms. Try again.
 
Not following the abbreviations either, but the voltages given, appear to be a floating neutral, with a hot shorted to ground somewhere. Is this an ups? Or feed from a step down transformer?
 
Maybe:
IOC input output cabinet where field terminations are made to device connected by PLC and SCADA cabling also ca.led remote IO
LAS lightning protection system or surge protector
 
My apologies. IOC & LAS as J explains above. Just confusing things but the main issue is my 120V power feed into my IOC fed from an electrical panel via a 15A breaker powering a 24Vdc power supply running the PLC in the IOC with a UPS, 2hrs capacity.

1st scenario:
Lost the 120V power at the LAS unit. Removed LAS and with the 15A breaker off at the panel, Hot/neutral read 67V, Hot/Gnd read 47V, and Neutral/Gnd read 117V. Tested again with LoZ meter and Hot/neutral read 0v, Hot/Gnd read 0V, but Neutral/Gnd still read 117V.
Why neutral/Gnd reading 117V. System has been running for 11yrs no issues.

2nd scenario:
LAS had an internal 15A fuse that was blown. Tested LAS input and output for shorts(ohms/continuity) and all good. Replace blown 15A fuse and re-installed LAS into IOC. The 24Vdc power supply unit and the UPS were totally isolated from the 120Vac. No other components on the circuit other than a small cabinet heater connected. Turned on 15A breaker and the LAS smoked immediately.
- LAS 15A internal fuse did not blow
- 15A breaker did not trip
- 2nd IOC (identical) 30ft away lost 120V power source. Found out 1hr later before the UPS died. The LAS on the 2nd IOC blew its 15A internal fuse. Replace and back on line.
- Why did the issue on IOC #1 cause the LAS on IOC #2 to blow its 15A fuse?
- IOC #1 and IOC #2 have individual 15A breakers and the electrical panel but on the same phase ???
 
Shuntme said:
Why neutral/Gnd reading 117V. System has been running for 11yrs no issues.
Click to expand...
With the floating neutral (unbonded), if a phase goes to ground, the ocp will not trip, and ground will become the same potential to neutral, but if an opposite phase also goes to ground, the ocp will trip. When you were getting the 60 volts, the neutral was floating, but there was no fault to ground from any of the phases at that time.
 
Whats actually providing the 120V source? a control transformer, flow thru UPS or is it a branch circuit breaker from a distribution panel?
Photos and or control drawing would help.
Sounds like you might have energized equipment ground or as @hillbilly1 says a floating neutral.
 
tortuga said:
Whats actually providing the 120V source? a control transformer, flow thru UPS or is it a branch circuit breaker from a distribution panel?
Photos and or control drawing would help.
Sounds like you might have energized equipment ground or as @hillbilly1 says a floating neutral.
Click to expand...
208/120V sub panel. UPS for the IOC is local to the IOC not to the system, besides it's been isolated from the 120Vac power.
 
hillbilly1 said:
With the floating neutral (unbonded), if a phase goes to ground, the ocp will not trip, and ground will become the same potential to neutral, but if an opposite phase also goes to ground, the ocp will trip. When you were getting the 60 volts, the neutral was floating, but there was no fault to ground from any of the phases at that time.
Click to expand...
The breaker off Hot/Gnd 67V and Hot/Gnd 48 V are proven to be phantom or induced voltages as they go to 0V when using a LoZ meter.
The Neutral to Gnd 117V is the mystery unless somehow/somewhere in the conduit/trunk whatever happened the neutral got blown out and shorted to a hot line(separate from my hot for this circuit) giving me 117V to Gnd with breaker off ?
I'm assuming if the phase grounds out then the 15A breaker(at panel) should trip but it did not. When I turn on the 15A breaker, there are no fireworks(shorted) and it now reads 117V to neutral? :confused: 🆘
 
Check continuity of the equipment grounding path from your control panel to the breaker panel. In a older building this may be a conduit type equipment ground. I have seen a loose lockout , broken coupling even a corroded conduit body, effectively open the equipment ground path.
 
FWIW, on my UPS battery back up the receptacles switch from bonded neutral to floating immediately on Power loss. A low Z meter would cause the 67 volts to disappear, not because it’s phantom but because it bonded one conductor to the EG. You don’t have a neutral once the UPS activates, you have two conductors, one of which happens to be white.
 
Shuntme said:
The breaker off Hot/Gnd 67V and Hot/Gnd 48 V are proven to be phantom or induced voltages as they go to 0V when using a LoZ meter.
The Neutral to Gnd 117V is the mystery unless somehow/somewhere in the conduit/trunk whatever happened the neutral got blown out and shorted to a hot line(separate from my hot for this circuit) giving me 117V to Gnd with breaker off ?
I'm assuming if the phase grounds out then the 15A breaker(at panel) should trip but it did not. When I turn on the 15A breaker, there are no fireworks(shorted) and it now reads 117V to neutral? :confused: 🆘
Click to expand...
Yes, a low Z meter would read zero with a floating neutral. That’s definitely what’s going on. The 15 amp hot shorting to ground with a floating neutral will not trip the breaker because there is no return path. Disconnect the load and neutral from that circuit, you will find you have continuity between the hot and ground on that circuit.
 
ptonsparky said:
FWIW, on my UPS battery back up the receptacles switch from bonded neutral to floating immediately on Power loss. A low Z meter would cause the 67 volts to disappear, not because it’s phantom but because it bonded one conductor to the EG. You don’t have a neutral once the UPS activates, you have two conductors, one of which happens to be white.
Click to expand...
Appreciate the insight but I don't think that's the case here as the UPS is totally isolated for the 120Vac lines. Come straight from the Electrical panel and open end wires in the cabinet and that's where I am measuring the 117V neutral to ground. I am going to start isolation sections(cutting blk. white, green and trace backwards to the panel to locate the short and replace the bad sections. Megged blk to ground and it is shorted. Neutral at the panel checked out solid so somewhere along the run something shorted out causing the problem, I think.
 
tortuga said:
Check continuity of the equipment grounding path from your control panel to the breaker panel. In a older building this may be a conduit type equipment ground. I have seen a loose lockout , broken coupling even a corroded conduit body, effectively open the equipment ground path.
Click to expand...

(y) Next step tomorrow.
 
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