# Shunt resistor continuous opperation; academic Q

#### ballsystemlord

Joined Nov 19, 2018
39
Hello,
My father recently encountered a problem. There is a pump drive that is very old and it recently started tripping the circuit breaker after about 30min of operation. After carefully examining the problem he discovered that the pump drive has a lower and lower efficiency as it ages. He increased the circuit breaker to 100A and the pump drive and pump now run fine (yes, he did check the pump and wire).
But it was difficult to observe the problem at first because it took several hours before it would occur.
I thought about this and wondered, as a purely academic question how this might have been observed from the beginning without having to sit there and observe the pump and pump drive.
A shunt resistor with a multimeter and a camera would have worked well I thought, but I've no idea how much voltage shunt resistors are rated for (we're talking a 3-phase to pump, 2-phase into pump drive circuit @ 240V), there's also the matter of derating which I've not yet had to do because my projects have all been low voltage and amperage DC.
I tried to search for this online but the only help I could find was wikipedia which simply said: "All shunts have a derating factor for continuous (2+ minutes) use, 66% being the most common, so the example shunt [ 500A 75 mV ] should not be operated above 330 A (and 50 m drop) longer than that..... [citation needed]"
That's not very clear (What about +2h? At what voltage?) and there's no reference to read either. Normally resistors have wattage and voltage rating, and after some searching of ebay, shunt resistors never seem to have one. Nor have I found one that explicitly states a derating factor.
Thanks

#### Yaakov

Joined Jan 27, 2019
3,168
A logging clamp meter would be better.

#### BR-549

Joined Sep 22, 2013
4,938
"My father recently encountered a problem. There is a pump drive that is very old and it recently started tripping the circuit breaker after about 30min of operation. After carefully examining the problem he discovered that the pump drive has a lower and lower efficiency as it ages. He increased the circuit breaker to 100A and the pump drive and pump now run fine (yes, he did check the pump and wire)."

I do believe your education has failed you and your father. You have not solved a problem. You have created a dangerous situation.

That is not the way to restore proper and safe operation.

Why did the pump lose efficiency? Bearing? Seal? Impeller? More head? Clogged lines?

#### ballsystemlord

Joined Nov 19, 2018
39
"My father recently encountered a problem. There is a pump drive that is very old and it recently started tripping the circuit breaker after about 30min of operation. After carefully examining the problem he discovered that the pump drive has a lower and lower efficiency as it ages. He increased the circuit breaker to 100A and the pump drive and pump now run fine (yes, he did check the pump and wire)."

I do believe your education has failed you and your father. You have not solved a problem. You have created a dangerous situation.

That is not the way to restore proper and safe operation.

Why did the pump lose efficiency? Bearing? Seal? Impeller? More head? Clogged lines?
The pump did not. Quoting myself, "...he discovered that the pump drive has a lower and lower and lower efficiency as it ages."
This is explicitly spelled out in the pump drives manual. The wire, pump, and box were both inspected for signs of failure or wear all the way back to the meter.
As for the possibility of there being additional gunk in the line there is a mesh filter on the intake side. That also was checked.
Thanks for the concern, though.

A logging clamp meter would be better.
Great idea. I aught to have mentioned, he has 2 of those, the problem is that they both auto-power off after 5-10min (I dislike things with auto-power-off).

#### WBahn

Joined Mar 31, 2012
26,398
I tried to search for this online but the only help I could find was wikipedia which simply said: "All shunts have a derating factor for continuous (2+ minutes) use, 66% being the most common, so the example shunt [ 500A 75 mV ] should not be operated above 330 A (and 50 m drop) longer than that..... [citation needed]"
That's not very clear (What about +2h? At what voltage?) and there's no reference to read either. Normally resistors have wattage and voltage rating, and after some searching of ebay, shunt resistors never seem to have one. Nor have I found one that explicitly states a derating factor.
Thanks
I don't understand what is not clear. It states that a common derating factor for operating a shunt continuously is 66% and that anything over 2 minutes is considered to be continuous operation.

As for whether a particular shunt publishes a derating factor for continuous operation is a separate matter.

I just did a Google search with the terms: shunt continuous operation, and found the following site as the very first nonmedical site:

http://deltecco.com/

Which states right on the main page, "For continuous operation, it is recommended that shunts are not used at more than 2/3 of the rated current under normal operation conditions. Shunts should be located in an area where freely circulating air is available. If this is not possible adequate forced ventilation should be provided to keep the shunt operating temperature at -40° to +60°C. Shunt temperature must be maintained under 145° C or a permanent change in resistance will occur."

So there is your real answer -- for their shunts, operate them under conditions that will keep the temperature below +60 °C and under no circumstances let them go about 145 °C.