Contact Rating of 24Vdc Inhibit Transfer Relay 7

[leur2011]

Is there a concern to look at for a low voltage automatic transfer switch (ATS) with inhibit transfer relays externally powered from a 24Vdc; while the 86 lockout relay 2.8A coil current at 125Vdc located at MCC use to block transfer and re-transfer the ATS in case fault occurs at the remote feeder LVPCB.

When fault occurs at the feeder CB, the device 86 at MCC will trip and lockout, and block transfer and re-transfer of ATS. In this scenario, would the contact rating of 24Vdc inhibit transfer relays are adequate to sustain 86 lockout relay 2.8A coil current at 125Vdc while the LOR is mechanically held at trip position?

 

[leur2011]

But if the contacts only list a 24V rating they're not going to do well at 125V

The 24VDC contact of relay will melt when block transfer and re-transfer operation of Automatic Transfer Switch due to the trip and lockout in the 125VDC 86 LOR. Would this be the case?

waross also pointed out there is issue of differing 24VDC and 125VDC relay coil voltage and failure to clear the switching arc and flash-over. When will be this specific switching event happen that I need to check?

 

[waross]

When current carrying contacts separate or open there is an arc. This may be a switch a push button or relay contacts.
When the relay tries to open and stop the current with a voltage above the rating, the arc may not extinguish.
The arc may pas enough current to keep the load energized. The heat from the arc will damage the relay. It may melt the contacts or it may melt or burn the relay frame.
This will happen when the relay tries to disconnect the load.
The 24 Volt relay should not be used to control the 115 VDC lockout relay.
However if the lock-out relay is switching the 24 Volt relay, there should be no issues.

Just to be sure I have checked the GE catalogue for the GE CR 420 N relay.
The maximum DC voltage allowed on the contacts is 24 Volts Direct Current.
[URL unfurl="true"]https://www.geindustrial.com/catalog/controlcatalog/10_CC.pdf[/url]

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[leur2011]

I am unable to read the diagram

I attached clearer picture of the control diagram.

When fault occur at feeder CB in SG-002, the 125VDC 86 LOR located at MCC will trip and lockout. The 125VDC 86 LOR will block transfer and re-transfer control operation of 24VDC inhibit transfer relay located at ATS.

 

[davidbeach]

That is just as unreadable as the first image.

 

[waross]

What David said, but readable enough to suspect that the one-line diagram does not have the information that we need, but unsure.
BUT

When fault occur at feeder CB in SG-002, the 125VDC 86 LOR located at MCC will trip and lockout. The 125VDC 86 LOR will block transfer and re-transfer control operation of 24VDC inhibit transfer relay located at ATS.

If this statement is accurate the 86 LOR should be able to safely switch the coil of the Transfer Inhibit Relay.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[leur2011]

That is just as unreadable as the first image

OK guys hopefully attached diagram is clear by now. Sorry for attaching truncated images.

 

[rcw retired EE]

Your diagram is a one line, not a control wiring diagram. It doesn't tell which way the control is going.

From my ATS experience and your first description of the issue, I surmise that the 86 lockout relay is tripped by the 480V breaker trip unit when a fault occurs on the ATS load. Lockout relay contacts then energize the block transfer relay in the ATS to inhibit the ATS from transferring the faulted load to the alternate source.

The 86 with its substantial contacts is controlling the 24V coil of the block transfer relay. Not the other way around. The ATS is receiving the "BLOCK TRANSFER," not sending it.

You may need to check the contact rating of the breaker trip contact that operates the 86. The auxiliary contacts on the breaker trips units are usually relatively light duty for alarm functions.

 

[stevenal]

A DC schematic is needed here.

 

[leur2011]

@rcwilson, thanks for describing clearly the control operation philosophy.

- When you said the lockout relay contacts (with 125VDC coil voltage) to energize the block transfer relay (with 24VDC coil voltage) in the ATS. Would that be OK. The 24VDC coil of inhibit transfer relay in the ATS is fed from dedicated external 24VDC power source as per the diagram.
- 10 Amps at 125VDC is the contact rating of 480V feeder CB trip circuit contact that operates the 86. No issue.

 

[LionelHutz]

Yes, you can have a control system that has both 125VDC and 24VDC that will work fine. Of course, you have to build it correctly.

The relay coils and contacts are separate items that are electrically isolated from each other. This means 125VDC and 24VDC can both be connected to the same relay and still be kept separate from each other and never connected together. Heck, you can have a relay with multiple isolated contacts and each of them could have a separate voltage so it could be possible to have many different voltages or power sources connected to a single relay.

I too expect that rcwilson is correct and a contact from the 86 relay will energize the block transfer relay. So, the 125VDC from the trip circuit will be wired to the COIL of the 86 relay. The 24VDC from the ATS control circuit will be wired to a CONTACT of the 86 relay. The voltages will remain isolated from each other just fine.

 

[leur2011]

Many thanks to waross, dpc, davidbeach, rcwilson, & LionelHutz for sharing your expert insight.

 

[waross]

On behalf of all of us, You are very welcome.
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter