north vault to south vault transfer switch

[joshgarcia]

In the power plant I work at we have a North and a South battery vault which feed a subpanel that provides the control power to our 12kv switchgear. There is a transfer switch to switch from N. to S. vault power. To avoid tripping the plant offline when I switched from South to North I installed jumpers across the N. feed to the load terminals (after checking my difference in potential) inside the transfer switch to prevent an interruption in power to the subpanel being it is not a make before break switch. There were spare lugs at each termination point making it both safe and easy. Now my supervisor wishes to install a disconnect or circuit breaker in line with this "jumper" to avoid having to work the circuit live inside the transfer switch when doing our vault swaps. My question is: do we need to size our "jumper circuit" to the vault overcurrent protection sizing which is 200A? I would think so but hoping to be able to order smaller and cheaper! I have never seen the amp meters show more than 8 Amps being drawn from the subpanel, but I know the potential is there.

 

[augie47]

If you have jumped out the switch, what purpose does the switch itself now play ?

 

[chris kennedy]

Gus, as I read the OP the switch is jumped out temporarily for no interruption of power during transfer. But I don't see how his supers idea will prevent live work.

 

[Don S.]

Installation of a breaker or switch in series with his jumpers will eliminate the need to install the jumpers for his north/south transition, and then remove them to return to normal operation. He can just close the breaker or switch, do the transfer, and then open the breaker or switch.

 

[brian john]

Installation of a breaker or switch in series with his jumpers will eliminate the need to install the jumpers for his north/south transition, and then remove them to return to normal operation. He can just close the breaker or switch, do the transfer, and then open the breaker or switch.

Which after the initial install minimizes exposure.

Size the circuit breaker and jumper to match the existing OCP and conductors.

 

[joshgarcia]

Exactly our intentions Don S. Thank you. So my question is do I need to size my conductors and switch or circuit breaker for the "jumper" circuit the same as my battery bank feeders and OCD which are sized for 200 Amps? I believe I should but hoping to save $ and use materials we already have by downsizing. Like I said before the ammeters from each vault have never indicated more then 8 - 10 Amps.

 

[joshgarcia]

Okay thanks alot Brian.

 

[__dan]

Use blocking diodes and fuses

Use blocking diodes and fuses

If that was my change, I would be very tempted to use blocking diodes and fuses in series with the tie breaker. The blocking diodes are arranged to prevent backfeeding DC to the other battery bank at lower voltage potential.

 

[joshgarcia]

Thanks Dan, that seems to be a good idea. Everytime before performing this swap I do check my difference in potential. It is always less then a 2 or 3 volt difference.

 

[SG-1]

Size the circuit breaker and jumper to match the existing OCP and conductors.

Agreed. If something bad does ever happen you will not be standing at an intentionally created weak link. Why not install a Test Switch to measure voltage ? It could be fused with a current limiting 1 Amp fuse. This would increase safety during the voltage measurements.