500v on a 480v system

[Dennis Alwon]

This was pm'ed to me by Remitts who works at a school

I'm Having an issue with my primary voltage is 500v on a 480v system, talked to the power company and they said it was because I was less than a mile from a substation. any recommendations on how to lower the voltage. I work in a school and I have seen that we have been replacing flat screen monitors alot in the last few months.

 

[hillbilly1]

The stepdown transformers feeding the 120/208 volt loads usually have adjustable input taps, so they could be adjusted for the higher input voltage.

 

[iwire]

The stepdown transformers feeding the 120/208 volt loads usually have adjustable input taps, so they could be adjusted for the higher input voltage.

For sure and I would point out that 500 is not really that high.

 

[hillbilly1]

I wonder if he may have a bonding issue on the LV transformer, if XO is not bonded and left "floating", and there is fault with one of the phases to ground, it could cause some issues. I would check all phases to ground and neutral for any abnormalities.

 

[Dennis Alwon]

I am curious as to what the monitor voltage is? If those are 120V monitors then the 480V is not the issue. Is there 2 services feeding the building or just 480V with step down tranies.

 

[hillbilly1]

I am curious as to what the monitor voltage is? If those are 120V monitors then the 480V is not the issue. Is there 2 services feeding the building or just 480V with step down tranies.

Usually(but not always), there is a customer owned step down transformer for the 120 volt loads, if the incoming voltage is high, the transformer output voltage will proportionally be high also. But as Bob wrote, that's not really that high, that's why I have a suspicion that the step down transformer is not properly bonded.

 

[iwire]

if XO is not bonded and left "floating", and there is fault with one of the phases to ground, it could cause some issues.

Could it hurt a TV or are you just assuming it would?

I am having a hard time imaging it would matter to to a TV.

Not a great safety wise but not sure it would kill the TV.

 

[hillbilly1]

I've found that problem plays havoc with electronics, with grounded power supplies, if one phase is grounded it puts 208 volts between the hot and ground if the tv or computer is on an opposite phase and blows the surge protection because it sees the high voltage to ground. I also had a now defunct electronics store where all of the stereos on display would vibrate on the shelf when they were one, transformer was not bonded, bonded transformer, they no longer vibrated.

 

[brian john]

I wonder if he may have a bonding issue on the LV transformer, if XO is not bonded and left "floating", and there is fault with one of the phases to ground, it could cause some issues. I would check all phases to ground and neutral for any abnormalities.

I doubt having a fault on an ungrounded system is going to cause operational issues. I have seen a few systems that had exactly this problem one was a SCIF (Sensitive Compartmented Information Facility) and the equipment in there was very sensitive and operated that way for a long time, with "A" phase grounded. What will happen if they have SPD is another issue.

Additionally the voltage at the monitors is 125 VAC L-N and 216 VAC Phase to Phase, assuming the transformer taps are set for 480 VAC.

 

[hillbilly1]

I doubt having a fault on an ungrounded system is going to cause operational issues. I have seen a few systems that had exactly this problem one was a SCIF (Sensitive Compartmented Information Facility) and the equipment in there was very sensitive and operated that way for a long time, with "A" phase grounded. What will happen if they have SPD is another issue.

Additionally the voltage at the monitors is 125 VAC L-N and 216 VAC Phase to Phase, assuming the transformer taps are set for 480 VAC.

You don't have to believe it, but I have seen it happen several times, just recently on several computer monitor power supplies. They replaced several that were on the same phase, they would blow as soon as they were plugged in. Line to neutral voltage was 120 volts, but line to ground was 208. Found the transformer was unbonded, corrected, found the short to ground. No more blown power supplies. Disassembled one of the power supply's and found the MOV's burnt between the line and ground input. (The ones one the same phase as the fault were unaffected)

 

[brian john]

You don't have to believe it, but I have seen it happen several times, just recently on several computer monitor power supplies. They replaced several that were on the same phase, they would blow as soon as they were plugged in. Line to neutral voltage was 120 volts, but line to ground was 208. Found the transformer was unbonded, corrected, found the short to ground. No more blown power supplies. Disassembled one of the power supply's and found the MOV's burnt between the line and ground input.

Well I had had the exact opposite experience and as noted the SCIF had very sensitive equipment.

 

[hillbilly1]

Well I had had the exact opposite experience and as noted the SCIF had very sensitive equipment.

But that doesn't mean it can't happen, because it has.

 

[brian john]

But that doesn't mean it can't happen, because it has.

I am not disagreeing with you. Read what I wrote CAREFULLY, SPD's? TVSS call it what you may.

 

[iwire]

You don't have to believe it, but I have seen it happen several times, just recently on several computer monitor power supplies. They replaced several that were on the same phase, they would blow as soon as they were plugged in. Line to neutral voltage was 120 volts, but line to ground was 208. Found the transformer was unbonded, corrected, found the short to ground. No more blown power supplies. Disassembled one of the power supply's and found the MOV's burnt between the line and ground input. (The ones one the same phase as the fault were unaffected)

I am not an electronics expert by any means but wouldn't the only connections between the EGC and circuit conductors be just the surge suppression?

 

[kwired]

500 volts is pretty normal on a 480 volt system around here - especially if little or no load is on the system.

Are you measuring this voltage when there is little load or when the demand is up?

If you are running high voltage because you are so close to substation, can they change any taps on the transformer to compensate?

 

[hillbilly1]

I am not an electronics expert by any means but wouldn't the only connections between the EGC and circuit conductors be just the surge suppression?

It's a little different than the surge protection were used to, it opens the power supply to prevent damage to the equipment, back when there was still TV repairmen around, you could get it fixed, now we just through stuff away.(actually its surge protection rather than suppression in this case)