Contact Resistance

Status
Not open for further replies.
tkb

tkb

Senior Member
Location
MA
I have some millivolt readings from across a circuit breaker and was wondering what would be a good reading and a bad reading.

2 pole 30 amp SqD QOB breaker in a PDU.
I have a reading from the buss to the load terminal of the same phase.
The readings I have are 26mv and 83mv.
Load is about 9.5 amps @ 120/208v

Are these mv values high or low?
What would be a bad reading?
 
Panel is live and the breaker is on.
A perfect connection should show 0 volts.
The millivolt reading is showing that the breaker has resistance.
The more resistance the higher the voltage across the breaker.
 
tkb said:
I have some millivolt readings from across a circuit breaker and was wondering what would be a good reading and a bad reading.

2 pole 30 amp SqD QOB breaker in a PDU.
I have a reading from the buss to the load terminal of the same phase.
The readings I have are 26mv and 83mv.
Load is about 9.5 amps @ 120/208v

Are these mv values high or low?
What would be a bad reading?
Click to expand...

NETA states that there is a concern when the readings are different by 50% or more. This is what you have. I'd call the manufacturere. Can you take ductor readings to corelate?
 
090427-2013 EST

tkb:

A couple quick comments.

Is the same current flowing thru each half of the breaker? Did you verify the same current?

It appears you are providing a voltage drop reading for each half of the breaker from its bus to output terminal. I do not know what a maximum value should be. On a post somewhere on this site I believe I indicated a measurement I made on a single pole breaker.

If you took the breaker out and used a large copper rod to serve as the bus connection, apply a low voltage across the breaker to develop the desired current, then you can measure the voltage drop from a point on the Sq-D clip, not the bus, to the head of the output screw. This voltage will be the internal voltage of the breaker. Basically the thermal element and the contact. This is known as a four terminal resistance measurement. This eliminates any variation due to contact resistance to the bus. You can do this at different current levels to see if there is a variation in the internal contact resistance of the breaker contact, and/or thermal element.

The resistances you measured are 26/9.5 = 2.7 milliohms, and 83/9.5 = 8.7 milliohms.

.
 
markstg said:
NETA states that there is a concern when the readings are different by 50% or more. This is what you have. I'd call the manufacturer. Can you take ductor readings to correlate?
Click to expand...

Small frame CB and the manufacture is going to say throw it away buy another one.

Typically I would think 26-40 millivolts would be expected at this voltage and current above that I would be concerned as this is a PDU I assume this is a critical load? I would recommend scheduling a time to replace.


IR and comparison reading as the best investigative tool for this type of work.

Ductoring small frame CB is typically very difficult depending on the brand and style, bolt in or snap in.
 
Last edited:
Status
Not open for further replies.
Top