Busbar & Tap off box - power loss & Over-voltage issues

[MarkyofftheMark]

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. Has anyone else come across this or had experience of this.

 

[davesparks]

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.

 

[MarkyofftheMark]

Thanks for this. I've checked the neutral on the 3 phase board and it seems in order.

 

[davesparks]

Thanks for this. I've checked the neutral on the 3 phase board and it seems in order.

But it’s the neutral in the tap-off and busbar which is damaged and causing the issue. This is obvious from the pictures you have posted.

 

[MarkyofftheMark]

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?

 

[Pete999]

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.

 

[MarkyofftheMark]

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.

 

[Pete999]

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?

 

[MarkyofftheMark]

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.

 

[Pete999]

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?

 

[Wilko]

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.

 

[MarkyofftheMark]

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.

 

[MarkyofftheMark]

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.

yes, tap off boxes already on order.

 

[Pete999]

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?

 

[Wilko]

...I've had to isolate the whole board as the lights were flashing and chargers plugged into sockets were smoking.

Hi - As @davesparks has said - the over voltage you are seeing must be from lost N. That has to be found and fixed.