Little design input

[buffalonymann]

I have XFMR 480:240 delta-delta. Installed 3 pole contactor to open 3 phases feeding resistive loads via solid state relays. The secondary side of the delta is not grounded, and I need to monitor the contactor poles so that when the contactor is opened, must verify there is no voltage on load side of contactor (in case on of the contacts weld closed). Initially thinking about putting 240VAC relays L1 to T2 - L2 to T3- L3 to T1. Problem is that if SSR is shorted current will flow through heaters and burn out relay coil. Was looking at Schneider CB but lowest value is 0.5A - the relay coil is 4mA. Looking for input

 

[petersonra]

I have XFMR 480:240 delta-delta. Installed 3 pole contactor to open 3 phases feeding resistive loads via solid state relays. The secondary side of the delta is not grounded, and I need to monitor the contactor poles so that when the contactor is opened, must verify there is no voltage on load side of contactor (in case on of the contacts weld closed). Initially thinking about putting 240VAC relays L1 to T2 - L2 to T3- L3 to T1. Problem is that if SSR is shorted current will flow through heaters and burn out relay coil. Was looking at Schneider CB but lowest value is 0.5A - the relay coil is 4mA. Looking for input

best bet is to just get contactors that have mechanically linked poles and monitor the auxiliary contact. IEC contactors are all made this way already. I don't know why it matters any that the secondary is ungrounded. That seems irrelevant to the issue.

 

[buffalonymann]

best bet is to just get contactors that have mechanically linked poles and monitor the auxiliary contact. IEC contactors are all made this way already. I don't know why it matters any that the secondary is ungrounded. That seems irrelevant to the issue.

If it were grounded, it would be simple to monitor for voltage. Are you suggesting that if one pole is hung up due to contact weld, the contactor armature will not seat in the off position, and therefore not release the aux contact?

 

[petersonra]

If it were grounded, it would be simple to monitor for voltage. Are you suggesting that if one pole is hung up due to contact weld, the contactor armature will not seat in the off position, and therefore not release the aux contact?

if you buy IEC style contactors they are all made that way.

 

[drktmplr12]

consider a diversified phase monitor.

https://www.marshbellofram.com/diversified-electronics/types/phasevoltage-monitors/

 

[petersonra]

If it were grounded, it would be simple to monitor for voltage.

Monitoring is not much different for grounded or ungrounded circuits.

 

[MrJLH]

Piece of advise, stay away from a underground delta.

 

[beanland]

Agree

Agree

Piece of advice, stay away from a ungrounded delta.

I agree. Options are to corner ground the delta, center side ground the delta (240/120V), or add a zigzag bank.

With only 3 wires and a 3-pole contactor, welding of ONE pole cannot be detected because the other two poles are open and no current can flow.

Ungrounded delta can lead to problems where spikes on the 480V side are capacitively (sp?) coupled from 480V winding to 240V winding. If you do not rate equipment accordingly, you can have random failures that are hard to chase down.