what constitutes a "high" neutral current?

[malachi constant]

Have a client (dispatch center) experiencing intermittent power quality issues (three ground faults trips in the first fifteen months, but not any in the last six months - I've posted on this in the past). They have a power quality meter in the gear that can log events. They recently set it to monitor four types of events: high power factor, low power factor, high neutral current, and unbalanced phase currents. They are getting 30+ "high neutral current" alarms a week. They have the "high neutral" setting at 1% of phase current. I assume this setting is way too low and is resulting in many false positives. What would you recommend they set the high neutral current alarm to? To put it another way, what would you say is a "normal" vs "abnormal" neutral current? I would say 1% is normal, 5 or 10% is abnormal, but I don't spend a lot of time looking at meters.

Thanks!!

 

[don_resqcapt19]

The code requires that the grounded conductor be sized based on the maximum unbalanced load. That would be with all of the line to neutral loads on one phase turned on and everything else off.

Without knowing a lot about all of the line to neutral loads there is no way to say what a high neutral current is. Depending on the conditions and the amount of line to line loads the normal neutral current could be 50% or more of the phase current.

 

[Smart $]

The code requires that the grounded conductor be sized based on the maximum unbalanced load. That would be with all of the line to neutral loads on [the] one phase [with greatest loading] turned on and everything else off.

...

Slightly modified... but I know what you meant.

PS: Code requires it to be the calculated neutral current of such, not the actual current. But the actual current would be the value to base the high neutral current setting on.

 

[texie]

Have a client (dispatch center) experiencing intermittent power quality issues (three ground faults trips in the first fifteen months, but not any in the last six months - I've posted on this in the past). They have a power quality meter in the gear that can log events. They recently set it to monitor four types of events: high power factor, low power factor, high neutral current, and unbalanced phase currents. They are getting 30+ "high neutral current" alarms a week. They have the "high neutral" setting at 1% of phase current. I assume this setting is way too low and is resulting in many false positives. What would you recommend they set the high neutral current alarm to? To put it another way, what would you say is a "normal" vs "abnormal" neutral current? I would say 1% is normal, 5 or 10% is abnormal, but I don't spend a lot of time looking at meters.

Thanks!!

If you are having GFP trips, I would be asking if the GFP was properly commissioned in the first place. The settings need to be appropriate for the application. Factors such as a neutral/ground bond down stream could have been there all along that are flowing current on the EGC but tolerated by the GFP due to it's setting but now maybe the current has increased. The first thing I would check is to see if there is any appreciable current flowing on the EG bus in the gear. Bottom line, if you are having GFP trips, there has to be a reason and the proper testing should be done to definitively find the cause if you want a safe a reliable system.

 

[Sierrasparky]

1% of phase current seems very low and most likely a default setting. You would need a perfectly ballanced system for less than 1 percent. Heck you would hardly need a nuetral.

 

[bob]

under certain conditions the neutral current would equal the phase current(100%)

 

[GoldDigger]

under certain conditions the neutral current would equal the phase current(100%)

In the case of non-linear (harmonic generating) loads, the neutral current can be higher than the current in ANY individual phase. Hence the need for oversized neutrals in some cases.
Without knowing the load it is hard to come up with a recommendation.

 

[Sierrasparky]

I would start with a setting close to the capacity of the un balanced load or nuetral.

 

[robbietan]

1% of phase current seems very low and most likely a default setting. You would need a perfectly ballanced system for less than 1 percent. Heck you would hardly need a nuetral.

I have seen some calculate 1% voltage unbalance = 8% to 10% current imbalance. and I hardly believe that the voltage unbalance for the system to be less than 0.5%.

In the case of non-linear (harmonic generating) loads, the neutral current can be higher than the current in ANY individual phase. Hence the need for oversized neutrals in some cases.
Without knowing the load it is hard to come up with a recommendation.

is a dispatch center the equivalent of a "call center" or BPO ? if it is, then I expect many computers and a load of harmonic currents, 3rd number.

 

[Sierrasparky]

I have seen some calculate 1% voltage unbalance = 8% to 10% current imbalance. and I hardly believe that the voltage unbalance for the system to be less than 0.5%.





is a dispatch center the equivalent of a "call center" or BPO ? if it is, then I expect many computers and a load of harmonic currents, 3rd number.

We are speaking of Nuetral currents not Voltage differences.

 

[GoldDigger]

We are speaking of Nuetral currents not Voltage differences.

Nevertheless, phase voltage differences can produce phase current differences, and in a wye configuration differences in phase current will usually result in non-zero neutral current.