Troubleshooting

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cavejumper

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Hello everyone, I don't post much, but I do come to the forum often and you guys are a valuable source of knowledge and experience for me! I know this is the place to ask a question when I can't get an answer anywhere else.

My basic question is this - When measuring voltage drop across a device such as a breaker, or fuse, or switch to check for loose/faulty components or terminations, what is an acceptable range? I usually work with 120-240VAC. I know 0.0V is ideal, and may be expected for a new install, but I'm concerned with devices that have been in use for some time.

Checking VD is part of my troubleshooting process for breakers and such. But I'm not sure where to draw the line between pass and fail for that test. In the past, I've replaced a main breaker that had a 5V drop between the line side lug and the load side busbar, and once I removed it, it was obvious that it was damaged. So I know that large of a VD is unacceptable. But my service call today was checking a fuse in an old fuse box, and I only measured 0.4V from the main line side lug to the load side screw on the fuseholder. But when there was a load on that circuit, the fuse was hot to the touch. I measured VD across the other fuses in that box, and they ranged from 0.1 to 0.3V, and none felt as hot as the first fuse did. I imagine that every situation will have different allowances, but I would appreciate feedback on what to be concerned about.
 
180530-1405 EDT

cavejumper:

Sometime back I measured the voltage drop across a QO-15, couldn't remember the results. Today reran experiment at 10 A. Drop was about 0.1 V. Thus, power dissipation is 1 W. At full rating I might expect somewhat more than 2.25 W because the thermal element is hotter, and resistance would be a little greater.

A higher current breaker should have a lower resistance, power input to the breaker is probably about the same for a range of breaker sizes. At 30 A I might expect about 0.05 V with a 20 A load.

You can experiment on the bench and see what results you get.

No way 5 V.

You are on the right track using voltage drop as a troubleshooting tool.

.
 
Both circuit breakers and fuses are thermal devices. They generate heat as part of their normal operation. They can get pretty warm sometimes. I don't think there is any VD across one that reliably indicates some kind of imminent failure. Essentially they act like a small electric heater. The more current the more heat and the more VD. The heat generated is a function of the square of the current so as current goes up heat goes up a lot more. VD goes up as a function of the square of the current as well. About all you can really do is check a known good unit under the same load and see what the VD is.
 
gar said:
180530-1405 EDT

cavejumper:

Sometime back I measured the voltage drop across a QO-15, couldn't remember the results. Today reran experiment at 10 A. Drop was about 0.1 V. Thus, power dissipation is 1 W. At full rating I might expect somewhat more than 2.25 W because the thermal element is hotter, and resistance would be a little greater.

A higher current breaker should have a lower resistance, power input to the breaker is probably about the same for a range of breaker sizes. At 30 A I might expect about 0.05 V with a 20 A load.

You can experiment on the bench and see what results you get.

No way 5 V.

You are on the right track using voltage drop as a troubleshooting tool.

.
Click to expand...

probably not 5V.
 
180530-1631 EDT

Knowing what voltage drop to expect across a particular good breaker in a good panel at a given load will allow you to spot problems with bad connections to or within the breaker like, high contact resistance between breaker and bus, poor contacts within breaker, insufficient contact pressure, and loose connection on output of breaker.

The voltage drop across the breaker is obviously load dependent.

.
 
cavejumper said:
once I removed it, it was obvious that it was damaged.
Click to expand...

IR scans and visual inspection, along with a noticeably hot area of a panel, or burnt electrical smell are usually sufficient to find a bad breaker, switch, receptacle, Etc.

There are cases when measuring voltage drop ia certainly good diagnostic procedure, however as a matter of standard troubleshooting, I rarely use voltage drop to find a problem... Many times, by the time you get there, whatever was hot has failed and opened up, or occasionally welded itself together causing a dead short.
 
Thank you everyone for your replies! It sounds like the best course of action when I'm troubleshooting is to just compare the suspect breaker with others in the same panel. I appreciate your insight into why VD measurements will change under differing circumstances, understanding that will help me interpret my test results.

I don't own an IR scanner, though maybe I should consider getting one. It'd be helpful for busbars and terminals, places I can't actually touch when they're energized.
 
180531-1154 EDT

cavejumper:

To a large extent you can consider the breaker and all its connections to be a resistance. Thus, voltage drop is roughly proportional to current thru the breaker, and you need to know that current to make judgements about the breaker.

Under failure conditions that resistance may not be linear or stable.

A 1500 W 120 V space heater is typically about 10 A when heated. This can be used as a convenient test load.

In my main QO panel and looking at a 20 A breaker, and using a 10 A heater I see a change in voltage from the breaker output screw head to neutral of 123.1 to 122.0 for the 10 A change in current. This is an impedance of 1.1/10 = 0.11 ohms. This is the pole transformer, service wiring, meter, and main fuses.

Looking direct across the QO-20 breaker, meaning from the QO bus bar to the breaker output screw, the change in voltage was 0.072 V. Thus, breaker impedance including its termination to the main bus was 0.072/10 = 0.0072 ohms. This is somewhat less than the 0.01 ohms I measured on the bench for a 15 A QO, and that is as expected.

If you can assume that a resistance or impedance is linear, then you do not need to start with 0 current as your base current for an impedance measurement. You simply need to know the change in voltage for a known change in current thru the impedance that the voltage is being measured across.

.
 
I don't know what the magic number is but would think you should expect at least less then a tenth of a volt drop across device or any other connection for that matter Doesn't matter what the system voltage is either, 0.1 volts with a 10 amp current will give up 1 watt of heat. If concentrated just on one tiny contact surface of say a 15 or 20 amp switch contact that is a lot of heat. 100 amp of current with 0.1 volts drop across a switch or connection is giving up 10 watts. It may not seem like that is much, but consider the small area it is concentrated on - it will overheat that junction.
 
most cb specs give Z at rated i
so measure v and i, calc z and see how close
the trend over time is as, if not more important than 1 reading

https://www.schneider-electric.us/en/faqs/FA173845/

https://testguy.net/threads/1369-Expected-contact-resistance-measurement-based-on-breaker-frame-size

I would expect less the 500 uohm
with 200-300 being new

so if you have 10 A you should see 5 mv or 0.005 v

good resource in general pg 64 in this case
http://www.iemworldwide.com/pdf/ansi-neta-ats-2009.pdf
 
I suggest you look into microhmeters. With those you pass a known DC current through breaker and measure contact resistance. Breaker must be removed to perform this testing and equipment isn’t cheap.

AC measurements can mask problems. Still what you are doing is of some value. Looking at resistance and wattage is the way to go. Any voltage drop needs to be on a loaded circuit and should be viewed as a % rather than a hard number.

For me, the device would matter. I might tolerate a 1 volt drop across a 3 phase 480 VAC NEMA size 2 motor starter with a load equal to 70% of its rating. I’d be a lot less inclined to tolerate it on the circuit breaker serving said motor starter.
 
Russs57 said:
I suggest you look into microhmeters. With those you pass a known DC current through breaker and measure contact resistance. Breaker must be removed to perform this testing and equipment isn’t cheap.

AC measurements can mask problems. Still what you are doing is of some value. Looking at resistance and wattage is the way to go. Any voltage drop needs to be on a loaded circuit and should be viewed as a % rather than a hard number.

For me, the device would matter. I might tolerate a 1 volt drop across a 3 phase 480 VAC NEMA size 2 motor starter with a load equal to 70% of its rating. I’d be a lot less inclined to tolerate it on the circuit breaker serving said motor starter.
Click to expand...
I'm not so certain about that. Breaker contacts and starter contacts - you want to see as little drop as possible across - thermal elements in the breaker as well as if the starter has thermal overloads will have some voltage drop across them. If you have electronic motor overloads but a thermal-mag breaker - you probably should expect more voltage drop across the breaker.
 
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