Phase loss monitor

[GlennH]

One of our hvac techs was sent to a facility to check a Carrier RTU that was not working. This was installed about a year ago with no problems until a storm came through and the unit stopped working. He found the phase loss monitor was not working, He had 240 volts between A-B, B-C, A-C, all were within 1 volt. No neutral, measured 240 volts to ground on two legs but 0 volts to ground on third. Facility electrician confirmed readings and said it was a corner grounded delta system. A replacement PLM was installed but it would not close either. When monitor was bypassed the unit ran fine with all readings normal.

I do not know the make or model number of the PLM but could someone confirm that it should work with this Delta system. Do they typically monitor to ground?

Thanks, Glenn

 

[GoldDigger]

A PLM way be made to work only with a wye system. Or, at greater cost probably, it can be made to work with wye, ungrounded delta or corner grounded delta. Without knowing more ( such as whether there is a fourth "neutral" input terminal) it is not possible to say.
It should never monitor to ground. It would be either three or four terminal, with the latter using neutral as the reference. Because of feed through back from loads it pretty much has to look at phase as well as voltage.

 

[Saturn_Europa]

That is fairly odd. Does the phase loss monitor, verify anything other phase loss? Most monitor: under voltage, over voltage, and phase reversal. Is it an adjustable model? The Tech may be able to fine tune it in.

Did the Tech check for phase reversal? I've run into one or two PM that I had to reverse the leads on them to get them to close. Or course check motor rotation before and after.

As a side note, we have plenty of pump stations that run on 240 V Delta that have PM.

 

[ptonsparky]

They should work on the Delta.

When installed they must be phased correctly.

The ones we use have an idiot light to make it simpler for this OF.

 

[Tony S]

They will work on corner grounded delta. All they do is check A-B, B-C, C-A are at the right voltage and in the correct order (phase reversal).

 

[Jraef]

Some will, some will not. It totally depends on what the PLM was purchased to do.
If they purchased one that is not designed to work on a corner grounded delta, then it will not.

RTFM...

 

[ptonsparky]

Some will, some will not. It totally depends on what the PLM was purchased to do.
If they purchased one that is not designed to work on a corner grounded delta, then it will not.

RTFM...

Or make a phone call because TFM does not give a hint.