Discuss Busbar & Tap off box - power loss & Over-voltage issues in the Commercial Electrical Advice area at ElectriciansForums.net

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On one of my sites, I have a 6 rising busbars ( E & I powerbar) rated at 250 A connecting a tap off box with a Schneider 100A isolator feeding a 3-phase board with 16mm SWA. I have around 24 tap off boxes onsite across 6 busbars and the installation is approximately 6 years old.
Over the past year I've had instances where the power has been completely lost. On shutdown and Inspection the busbar has burn marks and this has dropped out the phase; I've had to file the busbar and the tap off box connector to clear the residue in order to get this working.
This has happened to another box and the above works were carried out, but since we have had the voltage acting erratically and have been picking up 350V on two of the phases with just 50v on the third, which has burned out a significant amount of voltage sensitive equipment. The load on the board is well within the current carrying capacity and the SWA connections are secure on the tap off boxes. I've enclosed some pics to demonstrate. Busbar & Tap off box - power loss & Over-voltage issues Singeg Busbar - EletriciansForums.net Busbar & Tap off box - power loss & Over-voltage issues Singeg Busbar - EletriciansForums.net Busbar & Tap off box - power loss & Over-voltage issues Inside of tap off - EletriciansForums.net Has anyone else come across this or had experience of this.
 
I would hazard a guess that the damaged connection is on the neutral. The loss of the neutral to a three phase board will cause this rise in the voltage on some phases.

The busbar tap off unit will have been damaged by the overheating and the spring contacts will have lost their springiness.
By filing the busbar and the contacts you will have removed metal making the connection worse and causing this complete loss of the neutral.
 
the neutral in the Tap off box is secure on both sides of the isolator and there are no signs of damage to the cable. Could it be the neutral at the origin of the busbar is the issue, and if this is the case, why is only this tap off and one more affected on another run?
 
the neutral in the Tap off box is secure on both sides of the isolator and there are no signs of damage to the cable. Could it be the neutral at the origin of the busbar is the issue, and if this is the case, why is only this tap off and one more affected on another run?
Maybe this tap off is the one with the most load on it? without eyes on it's very difficult to form a proper opinion.
 
Absolutely, really appreciate your comments. The load on this board is no greater than any of the other 20+ boards. Has me slightly baffled. My thoughts are the tap off box connection wasn't great and its arced over time and caused the problem I'm experiencing now.
 
Absolutely, really appreciate your comments. The load on this board is no greater than any of the other 20+ boards. Has me slightly baffled. My thoughts are the tap off box connection wasn't great and its arced over time and caused the problem I'm experiencing now.
Are the blades showing signs of slackness? Does the DB this particular tap off feeds, have predominately SP&N circuits, or is it about the same as the rest of the install? Is the DB reasonably well balanced across all three phases?
 
The Tap off box fitted very securely to the busbar and doesn't jiggle or move when connected. The majority of circuits are SP & N with a couple of three phase MCB's.
 
The Tap off box fitted very securely to the busbar and doesn't jiggle or move when connected. The majority of circuits are SP & N with a couple of three phase MCB's.
Have you checked the current draw on the 2 phases, are there any big differences?
 
My 20p worth - once arcing has occurred in that tap off perhaps it should be replaced rather than repaired. Also, perhaps check the othe tap offs in the site (that are ok) are the same type.
 
I have monitored the current via the reader on the Schneider board and the current draw is at its peak around 25-30A which is consistent. I've had to isolate the whole board as the lights were flashing and chargers plugged into sockets were smoking.
 
I have monitored the current via the reader on the Schneider board and the current draw is at its peak around 25-30A which is consistent. I've had to isolate the whole board as the lights were flashing and chargers plugged into sockets were smoking.
As I said no eyes on, clutching at straws now, other than loose connections, who knows?
 
What is odd is the way the busbars have been connected to the live conductors viz: Brown, Blue, Black, a gap then Grey. This is wrong.

One would expect Brown, Black, Grey, a gap, then Blue so that the electromagnetic fields of the three lines combine to reduce in magnitude the resultant field around them. The Grey out on a limb to the right markedly stops this happening.

In the correct order Brown, Black, Grey a gap and then Blue, the effective situation is two parallel conductors ( one made up by the 3 lines and the other the neutral), with the current flowing in them being in opposite directions so the net em field produced by each current is minimised/cancelled out. This cannot happen as well in the actual set up shown in the pictures because the Grey is separated.

So, as well as the advice from others about dodgy connections the busbar ordering of live conductors exacerbates the electromagenetic effects of any transient currents. From Lenz's law rapidly changing strong EM fields around conductors induces high voltages. Perhaps this is the cause of the arcing.

You might want to investigate the nature of the loads on the failing tap off boxes and look for similarities between them not seen with the remaining tap off boxes.
 
Busbar & Tap off box - power loss & Over-voltage issues {filename} | ElectriciansForums.net


You ought to check whether the busbars are actually correctly connected to the lines and neutral but it is the tap off box which is incorrectly wired to the busbars ie: Black, Grey and Blue (- if Brown is correctly terminated you would expect to see the Black on the second from left and then Grey which your image does not show ). The isolator might be switching the neutral feed which is not intended. You'd get some odd voltages if this was the case AND A DANGEROUS SITUATION.
 
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It also looks as though those bus bars are tinned or plated to improve conductivity, filing the bars will have removed this.
 
Busbar & Tap off box - power loss & Over-voltage issues {filename} | ElectriciansForums.net


You ought to check whether the busbars are actually correctly connected to the lines and neutral but it is the tap off box which is incorrectly wired to the busbars ie: Black, Grey and Blue (- if Brown is correctly terminated you would expect to see the Black on the second from left and then Grey which your image does not show ). The isolator might be switching the neutral feed which is not intended. You'd get some odd voltages if this was the case AND A DANGEROUS SITUATION.

IGNORE MY #17. I have got myself addled with the above post but cannot edit it because 20 minutes has passed. What I want to say is go back to fundamentals and confirm the energisation of the busbars with respect to the three supply lines and neutral and then check how the connections at the top of the isolator have been made to them.
 
the neutral in the Tap off box is secure on both sides of the isolator and there are no signs of damage to the cable. Could it be the neutral at the origin of the busbar is the issue, and if this is the case, why is only this tap off and one more affected on another run?

It’s not the connections inside the isolator that I am talking about, it is the connection to the busbar itself. You can see burning on the busbar itself and not inside the isolator, therefore the problem is at the point at which is connects to the busbar.

The tap off will need to be replaced and moved to an undamaged part of the busbar. Fitting a new one in the same location will result in it burning out again due to the damaged busbar.
 

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