powerplay
Senior Member
- Location
- In front of a computer.
I put an Ohm Meter across a Breaker's Wire Terminal and the Bus Contact screw and found 3 ohms of resistance. Shouldn't an Breaker have pretty much zero ohms?
Resistance in the Breaker?
- Thread starter powerplay
- Start date
- Status
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
If you have disconnected the load wire from the breaker, you should indeed see a lower resistance. The ohmmeter can be confused by AC flowing through the breaker at the same time.
The most revealing measurement will be under a steady load, using a voltmeter across the breaker and a clamp ammeter to get the current. Then apply Ohm's Law.
The most revealing measurement will be under a steady load, using a voltmeter across the breaker and a clamp ammeter to get the current. Then apply Ohm's Law.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
141221-2355 EST
Why would you expect near zero ohms?
Second your measurement method is not an adequate method.
You need to do a 4-terminal resistance measurement. This is done by applying a known current thru the device to be measured, then measuring the voltage across two points on the device between which you want to know the resistance, and from voltage and current calculate the resistance.
See photos P19 thru P21 at http://beta-a2.com/EE-photos.html for 4-terminal resistors.
You can do your measurement in a powered main panel. Pick a circuit wih no load present. Connect a 1500 W portable heater to the circuit. After stablization measure the current thru this circuit.
Measure the voltage drop from the main panel bus bar near where the breaker connects to the bus bar to the circuit wire connecting to the breaker. This measures the breaker resistancre plus the contact resistance between the breaker and the bus bar, and also added is the contact resistance betwen the circuit wire and the breaker screw terminal.
Next put the voltage probes on the breaker terminals directly. This will show a slightly lower resistance.
As breaker size increases the resistance will drop.
The experiment can be run with a small breaker box and the breaker being tested on the bench. A little safer way to run the test. I just measured a QO20 in a small box where I can not do the complete test above. I can not get to the breaker terminal that clips on the bus bar. My measurement is from the bus bar to the breaker output terminal. I applied 7.9 A and the voltage drop was 0.119 V. Calculated resistance is 0.015 ohms. Power dissipation in 0.015 ohms at 20 A is about 6 W.
Resistance measured with a Fluke 27 is 0.2 to 0.3 ohms. So this is mostly probe contact resistance because of the 2-terminal measurement.
,
Why would you expect near zero ohms?
Second your measurement method is not an adequate method.
You need to do a 4-terminal resistance measurement. This is done by applying a known current thru the device to be measured, then measuring the voltage across two points on the device between which you want to know the resistance, and from voltage and current calculate the resistance.
See photos P19 thru P21 at http://beta-a2.com/EE-photos.html for 4-terminal resistors.
You can do your measurement in a powered main panel. Pick a circuit wih no load present. Connect a 1500 W portable heater to the circuit. After stablization measure the current thru this circuit.
Measure the voltage drop from the main panel bus bar near where the breaker connects to the bus bar to the circuit wire connecting to the breaker. This measures the breaker resistancre plus the contact resistance between the breaker and the bus bar, and also added is the contact resistance betwen the circuit wire and the breaker screw terminal.
Next put the voltage probes on the breaker terminals directly. This will show a slightly lower resistance.
As breaker size increases the resistance will drop.
The experiment can be run with a small breaker box and the breaker being tested on the bench. A little safer way to run the test. I just measured a QO20 in a small box where I can not do the complete test above. I can not get to the breaker terminal that clips on the bus bar. My measurement is from the bus bar to the breaker output terminal. I applied 7.9 A and the voltage drop was 0.119 V. Calculated resistance is 0.015 ohms. Power dissipation in 0.015 ohms at 20 A is about 6 W.
Resistance measured with a Fluke 27 is 0.2 to 0.3 ohms. So this is mostly probe contact resistance because of the 2-terminal measurement.
,
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
If the contact resistance of the breaker, properly measured, is 3 Ohms, then a 10A load will drop 30V across the breaker and dissipate 300W. The breaker would not last long under those conditions!
If you exersize the breaker that resistance will change as the breaker contacts actually rub together as the close.
Also, why are you even trying to measure contact resistance anyway? It is not a valid verification of a breaker. If you measured 3ohms across a contact that creates a voltage drop which equates to heat. You are measuring contact resistance with a very low voltage anyway. Are you experiencing a heat problem with the breaker?
Simply energize the breaker and when loaded measure the voltage across the line and load of that pole which will be the voltage drop. Using ohm law you can calculate the contact resistance by knowing that voltage and current, R=E/I.
Remember that a loaded breaker will generate heat as the BTUs that they generate are important to know when sizing the A/C for an equipment room. So, breakers naturally do generate a certain amount of heat. At one time I had a list of the various BTUs that I could proved designers so that they could size A/C units and/ of ventlation.
zog
Senior Member
- Location
- Charlotte, NC
You should use a microhmeter for that (Or voltage drop like templd mentioned), the value you expect to see depends on the rating of the breaker.
- Status