You all will like this one. We have contractors working on one of our buildings. Half on this building was remodeled 2 years ago and a new electrical room with new gear was put in. 3 days ago the fire panel and hvac control panel fried. I was asked to go check it out today. It was the E panel that had the problem. 480v breaker fed a small transformer that goes to a disconnet and then to a 120/208v E panel. The main bonding was done in the disconnect. The Main bonding jumper is a screw. The screw was not screwed in all the way and so the system was not bonded. Circuit 19 is what feeds the fire panel. All the breakers were turned off and when I measure phase to neutral it was 120, phase to ground was 120 and neutral to ground was nothing. When I turned on circuit 5 I had on circuit 19 phase to ground 347 volts, Phase to neutral 120 and neutral to ground I had 270. This is obviously the problem. I screwed in the Bonding screw all the way and the problem went away. What seems fishy to me is why now this problem shows up when the contractors started working on it and not earlier. Not trying to blame the electrical company but just tring to figure out what really caused it.
Loose Main bonding jumper
- Thread starter JdoubleU
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hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Something doesn't sound right, loosing the main bonding jumper should not have fried the alarm panel, the main bonding jumper only provides a short circuit fault current pathway for the grounding conductors and the case of the panel, there should have not been any loads on the grounding for this to occur, second the voltages you stated are impossible to obtain from the secondaries of a 480-120/208 3-phase transformer, it sounds like this transformer has been miss wired, it sounds like they are some wires feeding this transformer that have got crossed up, or the primary (480 volt) X0 is bonded to the transformer frame and the transformer is only bonded to the secondary and not back to the source feeding it, something is not right.
by the way you shouldn't have the primary X0 bonded, if you even have an X0 forget this if this is a delta primary.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
what do you mean the main bonding was done in the disconnect? is this disconnect on the 480 volt side? if so this is not the main bonding, as the main bonding has to take place on the secondaries of the transformer or at the first main disconnect, remember current only flow back to the source, and evey transformer will be a new source, so a ground fault after atransformer will only be seen as a line to line fault on the 480 volt side of it, so there also needs to be a main bonding in the 120/208 panel, and if the transformer is a Y primary then you do not bond the X0, but run an EGC from the source supply, you should not have a neutral ran to it from the source either.
What and how is bonded?
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When I tighten up the screw it fixed everything though.
The primary is a delta and the secondary is a y. The bonding is done in the first means of disconnect on the secondary side.what do you mean the main bonding was done in the disconnect? is this disconnect on the 480 volt side? if so this is not the main bonding, as the main bonding has to take place on the secondaries of the transformer or at the first main disconnect, remember current only flow back to the source, and evey transformer will be a new source, so a ground fault after atransformer will only be seen as a line to line fault on the 480 volt side of it, so there also needs to be a main bonding in the 120/208 panel, and if the transformer is a Y primary then you do not bond the X0, but run an EGC from the source supply, you should not have a neutral ran to it from the source either.
What and how is bonded?
I am trying to make since of this my self. You are saying that even though the voltage between the phase conductor and ground reads 347 that this still should not have fried the fire panel.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
it might look like it did, but not knowing where this voltage came from and now the bonding screw shorting it out, could be causing extra current on something else, I had a transformer that a previous contractor had replaced 3 times because it kept over heating, they would just hook it back up the same way it was installed the first time, they checked the current on the primary ungrounded conductors and thought all was ok, it wasn't tripping breakers and they just thought it was a faulty transformers they were getting, the manufacture Jefferson Electric who has call me before to check out warranty claims for them had me check out what was causing it after the 4th one burned up, and I found the X0 was bonded on the primary side, this contractor didn't even know he didn't need a Y primary, he thought since the service was a Y he had to use a Y to go from the 480/277 to the 208/120, well it was a mess, bonding was done wrong, no secondary bond, he bonded the primary side only, so there was no fault path on the secondary, and when you reported the voltage readings without the bond on the secondary, it through a few red flags in my mind as these voltages are not possible from the secondary side of a transformer unless there is some miss wiring.
Like I said, there are problems that are now just hidden by the bonding screw, and I would lay odds that you now have current on your grounding.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
no it shouldn't, the fire panel should not have any circuits referencing the grounding, all the loads should have been between the hot and neutral, loosing the main bonding jumper just in effect made this an ungrounded system, but also being an ungrounded system there should have not been a voltage there? tell me what would cause a voltage with enough current to damage a load be coming from? I can see stray voltage from induction or capacitive coupling, but as soon as any load was applied it would disappear, this is why we use loaded coil voltage meters to prevent a false positive in the reading, a DVM will not do this unless it has a circuit loading feature.
But if this voltage has enough current to do damage, you have some problems some where, which would require some mapping out of the affective circuits to find where its comming from, transformers isolate, so the secondary voltage is all you should ever see on any secondary side of a transformer, and if the readings are true loaded meter readings.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Wish you were somewhere around here, I love to trouble shoot these kind of abnormalities. it builds my referance as to what can cause what.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
That sounds like you forced the EGC to carry the secondary neutral current.
K8MHZ
Senior Member
- Occupation
- Electrician
Sometimes there is a difference between fixing things and doing something that makes a problem appear to go away.
I checked the current on the ground and it read .11 amps. My thought now is that is the 2 years we didn't have a phase to ground fault. The electrical contractors said that when this happened one of the trades drilled through a conduit and caused a fault. Now if this fault was a ground fault and the main bonding jumper was not connected to clear the fault could this have possible fried the fire panel.
brian john
Senior Member
- Location
- Leesburg, VA
If the bond screw was not connected and you have a fault on one phase you now have a grounded system where the voltage is (assume B phase is shorted)
A to ground 208 VAC, A to neutral 120 VAC
B to ground "0" VAC, B to neutral 120 VAC
C to ground 208 VAC , C to neutral 120 VAC
A-B 208 VAC
B-C 208 VAC
C-A 208 VAC
But no problem as far as operating equipment.
Now if the bond screw connected during this period there would be a operation of the OCP.
Unless they pulled off the neutral in the panel Not sure why those control panels fried. What if those panels had a floating ground.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
With the voltage readings in your original post, I'm leaning towards a 480/277 circuits mixed in the same conduits as the 208/120 volt circuits and somewhere a ungrounded 480 circuit is making contact with a 208/120 system hot maybe from circuit 5 or 19 , thus causing the strange voltages to appear between the neutral and ground of the 208/120 volt panel without the main bond, I can only explain this with the reading of 270 volts which could be a 277 volt reading with a voltage drop of 7 volts, and it seems to be associated with circuit 5 or 19? too tired to draw this out now, but as soon as I can I'll try so maybe I can see where it might be coming from.
one question, is there any 480/277 that could be associated with those two circuits?
brian john
Senior Member
- Location
- Leesburg, VA
If the load had a hot and a neutral, grounded or not (floating) would not effect it's operation.
Brian thank you for your help. After drawing it up in my head that makes since to me. I had a meeting with the electrical contractor the other day and explained to them that we essential had a ungrounded system and that there had to have been and event that occured that would put a high voltage between the phase and neutral. When I looked at the fire panel the fried points were on the phase and neutral terminals not on the ground. The contractors hired an engineer to prove to me that the system not being bonded is why the panel fried. His idea just didn't have me convinced. With the bond off we still had 120 between phase and neutral. He agreed with me that the only way this panel could have fried is because an event had to have happened that caused the voltage between phase and neutral to be to high. My thought is that another phase from another source got injected into that circuit. The engineer is off to do some research to prove that is was the bonding issue alone that caused the problem.
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