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?

 

[waross]

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

 

[dpc]

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.

 

[davidbeach]

I have proven this.

Do tell.

 

[leur2011]

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.

 

[waross]

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

 

[leur2011]

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.

 

[davidbeach]

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.

 

[waross]

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

 

[leur2011]

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.

 

[waross]

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

 

[mgtrp]

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

 

[leur2011]

@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.

 

[leur2011]

@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.

 

[waross]

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

 

[leur2011]

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

 

[NCTHAI]

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

 

[waross]

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

 

[leur2011]

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 ?

 

[davidbeach]

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.