Voltage readings today....

[Mule]

I was asked to assist a maintenance man today on a 3ph 3wire meter loop and a 7 1/2hp combination starter on a pole....

Here's the voltage readings...Starter is open (no load)

Ph A to B 500v
Ph B to C 500v
Ph C to A 500v

Ph A to Grnd 494
Ph B to Grnd 494
Ph C to Grnd 12v

Poco's secondary is Delta.....no reference to ground that I can see

What's going on? Do I have a accidental ground on the customers side?

 

[Strahan]

With it being an ungrounded delta readings to ground will not be "true" but indications are that you have a ground fault on phase "C". Start looking at the wiring for possible grounded conductor.

 

[Mule]

That's what Im thinking.....Two hunches....first it has a surge arrester hooked across the line side of the disconect......second...the conduit between the meter and the starter is really small...3/4 emt with two "slay's" mounted lid down with all three phases and a ground inside...little tight wouldnt you say?

 

[Strahan]

That's what Im thinking.....Two hunches....first it has a surge arrester hooked across the line side of the disconect......second...the conduit between the meter and the starter is really small...3/4 emt with two "slay's" mounted lid down with all three phases and a ground inside...little tight wouldnt you say?

I personally would suspect the surge arrester what ever it is it is a high resistance ground fault so I wouldn't lean towards wire so much as I stated earlier. I would have concentrated on the motor but since you stated you measured these values without a load that kinda rules that idea out. Good luck

 

[Mule]

I personally would suspect the surge arrester what ever it is it is a high resistance ground fault so I wouldn't lean towards wire so much as I stated earlier. I would have concentrated on the motor but since you stated you measured these values without a load that kinda rules that idea out. Good luck

Im thinking it would not nescissarily be a high resistance path.....If it was, it wouldn't shove the voltage so low on C phase ? Am I thinking wrong?

 

[quogueelectric]

Im thinking it would not nescissarily be a high resistance path.....If it was, it wouldn't shove the voltage so low on C phase ? Am I thinking wrong?

I am thinking it is a corner grounded delta.

 

[Strahan]

Im thinking it would not nescissarily be a high resistance path.....If it was, it wouldn't shove the voltage so low on C phase ? Am I thinking wrong?

I believe you are thinking right i was thinking more in the lines of current we know if it was a dead short to ground the current would be extremely high. High enough to trip your OCPD. Fault current would be E/Z depending on the resistance of conductors and bonding conductor which should be low your fault current will be extremely high.
Without knowing conductor size and length this can only be assumed. Thats why I say a high resitive(impedance) ground fault. Assuming the OCPD is sized right and you are not tripping it one could assume the current draw do to this short is reasonably low.

 

[quogueelectric]

Once more "Corner grounded Delta" Open the service up and see if it is bonded to a phase leg.

 

[Cold Fusion]

Poco's secondary is Delta.....no reference to ground that I can see

Just a suggestion. You may want to look at that again. I've never seen a 480V poco service that was ungrounded. There is probably one out there - I just haven't seen it.

First POCo supplied, corner grounded 480V delta I ever saw (early 70s - it was likely 20 years old then) had this bare #10 snaking down the side of the main fused disconnect, lugged to the box, then out the bottom of the box to a ground rod.

Can you open the main and get to the line side? If you can, open the main and measure the line side voltage.

cf

 

[Strahan]

Once more "Corner grounded Delta" Open the service up and see if it is bonded to a phase leg.

You may be right opening up the service or distribution center is not always easy and convenient shed the load and take readings again even if it is a corner grounded delta you should not read these type of voltages.

 

[Cold Fusion]

the conduit between the meter and the starter is really small...3/4 emt with two "slay's" mounted lid down with all three phases and a ground inside...little tight wouldnt you say?

7.5hp is the only load? What's that take? 3 - #14, maybe not even an EGC if the wiring method is metalic conduit.? 3/4 could be fine.

cf

Whoops - missed quoque's post. What he said first.

 

[Strahan]

Once more "Corner grounded Delta" Open the service up and see if it is bonded to a phase leg.

Alright quick let me take that back "corner grounded delta" never seen one I was thinking high leg delta crap

 

[quogueelectric]

Alright quick let me take that back "corner grounded delta" never seen one I was thinking high leg delta crap

It goes against all grain of common sence to an electrician but they are out there and mystify many. One of my first was a 5000a open buss at 480 in a powerhouse by me where a winding on a genny grounded to frame so they corner grounded a phase. I am standing in the middle of 5000a open buss reading 480/480/0 volts to ground freaky to say the least.

 

[Strahan]

Now my brain is spinning could someone point me to a good place for info on a corner grounded delta such as advantages disadvantages etc... I know they are no longer being installed but I would still like to understand them in the event I encounter one. Thanks

 

[iwire]

Now my brain is spinning could someone point me to a good place for info on a corner grounded delta such as advantages disadvantages etc... I know they are no longer being installed but I would still like to understand them in the event I encounter one. Thanks

They are still being installed.

If you have a delta source you have two options allowed by the NEC, corner ground it, or leave it ungrounded.

As far as Mules question.

It could be corner grounded delta and the 12 volts he is measuring could just be voltage drop on the grounded conductor.

OR

It could be ungrounded delta that has a ground fault and some resistance in the fault path.

 

[Mule]

7.5hp is the only load? What's that take? 3 - #14, maybe not even an EGC if the wiring method is metalic conduit.? 3/4 could be fine.

cf

Whoops - missed quoque's post. What he said first.

Im thinking I saw #8 or #6 with really thick insulation inside the cabinet coming from the meter.... sorry forgot to mention that....anyway I supspect a possible short since its so cramped...

 

[Mule]

Just a suggestion. You may want to look at that again. I've never seen a 480V poco service that was ungrounded. There is probably one out there - I just haven't seen it.

First POCo supplied, corner grounded 480V delta I ever saw (early 70s - it was likely 20 years old then) had this bare #10 snaking down the side of the main fused disconnect, lugged to the box, then out the bottom of the box to a ground rod.

Can you open the main and get to the line side? If you can, open the main and measure the line side voltage.

cf

They are still being installed.

If you have a delta source you have two options allowed by the NEC, corner ground it, or leave it ungrounded.

As far as Mules question.

It could be corner grounded delta and the 12 volts he is measuring could just be voltage drop on the grounded conductor.

OR

It could be ungrounded delta that has a ground fault and some resistance in the fault path.

I called Poco on the cell phone while I was looking at the bank, they verified, and I looked at the connections "no intentional ground" only to the tranformer tubs, the bottom side of the lighting arresters, butt wrap, and service drop tension wire, was the only connections.

So Here's the rest of the story....

after talking to poco and taking the readings, Im standing there with my gloves on, talking to the mainenance man. We were talking about a previous line fuse failure where poco re-fused. I suggested that it may have been lighting...then I suggested that he replace the surge arrester as a precaution, then he said..."Oh this one? and gently slapped the side of the small surge arrester with the back of his hand..(he had leather gloves on) ...then I heard the wires rattled in the riser pipe and it blew a fuse on the pole. He was nearly in shock....I told him do not touch anything!! Its old and dangerous... We called poco to come pull all of the fuses so we can troubleshoot, and that's what we are waiting on now. So I just thought I would bounce it off of you guys, to see if Im on track with possible probabilities........

So, Im going back out...Im thinking I will pull the meter, and look for continuity from C phase and the raceway and will find it either be upstream of the meter or downstream to the disconect, which will be the arrester, or the small conduit I mentioned...

 

[Strahan]

They are still being installed.

If you have a delta source you have two options allowed by the NEC, corner ground it, or leave it ungrounded.

As far as Mules question.

It could be corner grounded delta and the 12 volts he is measuring could just be voltage drop on the grounded conductor.

OR

It could be ungrounded delta that has a ground fault and some resistance in the fault path.

So the high leg delta configuration is not permitted by the NEC???? And as for the corner grounded system I'm pretty sure I read somewhere that these systems are nolonger being installed I'll try and find it

 

[iwire]

So the high leg delta configuration is not permitted by the NEC????

OK 3 choices, I don't see high leg services only Wyes.

And as for the corner grounded system I'm pretty sure I read somewhere that these systems are no longer being installed I'll try and find it

Perhaps your power company will not provide it but it is done with separately derived systems.

 

[Strahan]

OK 3 choices, I don't see high leg services only Wyes.




Perhaps your power company will not provide it but it is done with separately derived systems.

Wow we have many 240v high leg deltas. Wyes for lighting many of those to.
Yea I would agree with that but other that fault protection of the other two phases and many less expensive I'm still not seeing why.