Contact Rating of 24Vdc Inhibit Transfer Relay
Contact Rating of 24Vdc Inhibit Transfer Relay
(OP)
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?
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?






RE: Contact Rating of 24Vdc Inhibit Transfer Relay
I have seen DC relay contacts that didn't clear the arc. A small persistant arc eventually melted the silver alloy contacts which flowed together and completed the circuit. With no arc the molten silver alloy solidified into a miniature hour glass shape. Of course the brake never did apply.
Bill
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"Why not the best?"
Jimmy Carter
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
Before resetting the 86 LOR, I understand there are specific steps that must be taken to determine and isolate the cause of the roll out of the 86 device.
My question was focus on the 24Vdc dry contact rating of inhibit transfer relays if rated to sustain the prolong 2.8A coil current of 125Vdc 86 lockout relay while mechanically held at trip position. Does the dry contact rating of 24Vdc relay is suitable for the making and breaking i.e. to block transfer and re-transfer operations.
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
If it is a relay with a 24 Volt coil, We don't know.
Bill
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"Why not the best?"
Jimmy Carter
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
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 that 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.
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
Bill
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"Why not the best?"
Jimmy Carter
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
- Relay contact rating data = SPDT inductive (p.f.=0.4) 7A at 24Vdc. Resistive (p.f.=1.0) 15A at 24Vdc.
- Coil = 24Vdc, 650 Ohms, nominal current 36.9 mA at 60Hz.
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
"You are looking for serious issues if you try to switch 125 VDC with rely contacts rated at 24 VDC."
Bill
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"Why not the best?"
Jimmy Carter
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
BTW, below is the data of the 86 LOR. General Electric (GE) series 95.
The problem started when our contractor purchased and delivered at site a GE Zenith ATS with inhibit transfer relays externally powered from a 24Vdc. I am trying to figure out if poses serious safety issue.
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
Now it seems that this may not be the case.
I am confused.
I am unable to read the diagram.
Bill
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"Why not the best?"
Jimmy Carter
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
NC
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
Zero.
Undetermined.
It is not an issue of current.
Rather it is an issue of voltage and failure to clear the switching arc and flash-over.
Bill
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"Why not the best?"
Jimmy Carter
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
What are the two different issue of relay coil and contact ?
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
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?
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
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.
https://www.geindustrial.com/catalog/controlcatalog/10_CC.pdf
Bill
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"Why not the best?"
Jimmy Carter
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
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.
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
BUT
If this statement is accurate the 86 LOR should be able to safely switch the coil of the Transfer Inhibit Relay.
Bill
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"Why not the best?"
Jimmy Carter
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
OK guys hopefully attached diagram is clear by now. Sorry for attaching truncated images.
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
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.
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
- 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.
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
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.
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
RE: Contact Rating of 24Vdc Inhibit Transfer Relay
Bill
Bill
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"Why not the best?"
Jimmy Carter