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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?

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

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

2
Don't confuse the operating voltage of the relay COIL with the rated capability of the relay CONTACTS. Two completely different issues, and frankly I'm not sure which you are talking about. Bill is right that the lockout relay coil is a substantial inductive load, so beware. But normally, contacts on the LOR will interrupt the current flowing to the coil. If they don't, the LOR coil will quickly self-destruct. I have proven this.

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

Quote (dpc)

I have proven this.
Do tell.

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

(OP)
Warross, DPC, and davidbeach thanks a lot so far for all your valuable inputs.

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

It it is a 24 Volt relay, NO.
If it is a relay with a 24 Volt coil, We don't know.

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

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

(OP)
It is a 24Vdc coil voltage S.P.D.T relay module inside ATS. See below electrical diagram.

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

Every lockout relay (86) that I've ever seen interrupts the tripping current as soon as the lockout is in the tripped state. The coil wouldn't last very long otherwise.

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

There is no way we can answer your question without knowing the contact rating of the 24 volt relay.

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

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

(OP)
OK, it is a General Electric (GE) general purpose plug-in relay model CR420N.

- 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

From my first post;
"You are looking for serious issues if you try to switch 125 VDC with rely contacts rated at 24 VDC."

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

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

Maybe I'm misunderstanding the situation, but why is the transfer inhibit relay controlling the lockout relay, rather than the other way around?

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

(OP)
@waross, appreciate if you could explain further about the serious safety issue of the design. Is it because the different voltage rating of the inhibit transfer relay and the 86 lockout, or other concern.

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

(OP)
@mgtrp, you are correct. The operation of the control circuit is that when fault occurs at the feeder CB, the device 125Vdc 86 LOR in MCC will trip and lockout; and block the transfer and re-transfer operation of the 24Vdc inhibit transfer relays located at ATS.

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

Quote (OP)

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?
This is the question that I answered.
Now it seems that this may not be the case.
I am confused.
I am unable to read the diagram.

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

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

(OP)
@waross, thanks for your response, and yes that is the point i want to be clarified. You deserved an star.

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

To clarify my understanding, The relay has capacity to interrupt the 7A at 24 V DC. What is the maximum current that relay can interrupt without any safety risk at 125V DC? If this is correct, may be relay catalogue should have answer. If not, need to contact OEM.

NC

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

Quote (OP)

What is the maximum current that relay can interrupt without any safety risk at 125V DC?
None.
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
--------------------
"Why not the best?"
Jimmy Carter

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

(OP)

Quote ((dpc))

Don't confuse the operating voltage of the relay COIL with the rated capability of the relay CONTACTS. Two completely different issues.

What are the two different issue of relay coil and contact ?

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

It is quite possible to have a relay with a 24V coil that has contacts rated for 125VDC, if that's what's specified and ordered. But if the contacts only list a 24V rating they're not going to do well at 125V.

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

(OP)

Quote (davidbeach)

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?

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

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.
https://www.geindustrial.com/catalog/controlcatalog/10_CC.pdf

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

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

(OP)

Quote (waross)

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.

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

That is just as unreadable as the first image.

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

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

Quote (OP)

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

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

2
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.

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

A DC schematic is needed here.

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

(OP)
@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.

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

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.

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

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

RE: Contact Rating of 24Vdc Inhibit Transfer Relay

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

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

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