Non-preplanned motor current moniitoring and EF criteria

[GeorgeB]

I am involved with designing, testing, commissioning, and optimizing electrohydraulic systems.

Often, I want to instrument (monitor current) the electric motor (usually in the 5-200HP range) while I monitor, record, and make hydraulic changes to pressures and flows. At some of my more up-to-date customers who are aware of OSHA's electric flash issues, I've been forbidden to open panels (230, 480, and 600V) or MCCs to connect my "amprobe type" CT to give me that information.

Other than shutting down and installing a permanent CT, how are others accomplishing this function? Are there approved methods utilizing clamp-on CTs?

 

[H.L.]

Re: Non-preplanned motor current moniitoring and EF criteria

NFPA 70E allows live work for the purpose of testing that cannot be accomplished on a dead circuit. Of course appropriate PPE is required as is an energized electrical work permit. For samples of work permits Google "energized electrical work permit".

Hale

 

[charlie tuna]

Re: Non-preplanned motor current moniitoring and EF criteria

"B",
why not invest in a data logger instrument where you can install prior to adjustments and monitor all aspects of the motor's operation --voltage -amperage -kw-power factor? after the adjustments and testing is complete - shutdown and remove the instrument!

 

[GeorgeB]

Re: Non-preplanned motor current moniitoring and EF criteria

I emphasize that my comments below are not contradicting the excellent advice; the limitations are MINE, not the system's.

Charlie said: "why not invest in a data logger instrument where you can install prior to adjustments and monitor all aspects of the motor's operation --voltage -amperage -kw-power factor? after the adjustments and testing is complete - shutdown and remove the instrument!"

Charlie, there are 2 problems that I have with that. First, the value I bring to the customer is too little to justify the whole 9 yards for the multiple motors I usually will look at in a single day.

Second, for the existing installations, (working with an OEM on new product development is far less restrictive) I often go into process plants where shutting down to make my connections is not possible. Thus I am looking for a way to make and break while under power. As we don't usully know which are going to give trouble in advance, and absolutely cannot afford to fully instrument all of them full time. For maintenance, getting voltage for phase shift just isn't one I've been able to do.

As HL says, "NFPA 70E allows live work for the purpose of testing that cannot be accomplished on a dead circuit. Of course appropriate PPE is required as is an energized electrical work permit.". Yes, the problem I have is that I logically determine that I need to do something during diagnosis / hydraulic testing. The last 2 plants require over 24 hours to get the energized work permit, and then do not allow a fluid power guy with electrical knowledge in the panels (with which I agree, in general). I hope that the MCC manufacturers will come up with an insulated conductor that is accessible w/o exposure to an uninsulated conductor area.

Thanks, guys!

 

[petersonra]

Re: Non-preplanned motor current moniitoring and EF criteria

I think you are out of luck.

Doing what you want to do in a safe way is going to require certain precautions.

Think of it this way, if you manage to get into an MCC, and cause an arc flash, not only will you damage yourself but you will affect production as well. You might be willing to accept the liability on harming yourself (even though you shouldn't and they should not let you) but are you willing to take financial responsibility for the damage you might cause, and the subsequent loss of production?

 

[GeorgeB]

Re: Non-preplanned motor current moniitoring and EF criteria

"I think you are out of luck.

Doing what you want to do in a safe way is going to require certain precautions."

WHOA, I am not, repeat NOT, suggesting doing things that are unsafe. What I want is a SAFE way to get an "amprobe" type CT on a T lead, and am asking if anyone knows of one. I've (yes, dangerous) opened p%$#&^rheads (not sure that slang is appropriate) to do it, opened pull elbows and boxes to do it, and often gone into industrial panels and MCCs to do it. As I do not make or break connections, nor connect to hot wires, and have insulation (wire and CT) rated for the voltage between my equipment and the current carrying conductor, I was (and am) comfortable. Only if the contactor were opened (or cloesed) while I was inside would there be, as I see it, flash danger. It is easy for me to tell my customer that if he can get me SAFE and LEGAL access, I can give him more valuable information.

I make online connections to 400+ bar (6000+psi) hydraulic systems. I charge and check accumulators, using 6000 psi nitrogen bottles. None of these are unsafe, DONE PROPERLY. Putting a CT on a current carrying conductor also requires doing it properly ... output always with a load, although that isn't as much of an issue with the "amprobe" style. It is the ACCESS TO the INSULATED current carrying conductor that is difficult.

"Think of it this way, if you manage to get into an MCC, and cause an arc flash, not only will you damage yourself but you will affect production as well. You might be willing to accept the liability on harming yourself (even though you shouldn't and they should not let you) but are you willing to take financial responsibility for the damage you might cause, and the subsequent loss of production?"

I 100% agree with you. Indeed, an issue here, to me, is that I cannot reclose the door with my equipment connected after going through appropriate procedures; it is all fully insulated, and I am confortable that I won't CAUSE a flash. But a piece of zip cord (150V insultation?) hanging out of a gasketed (typically NEMA 12) door bothers me far more than snapping my sensor around a current carrying lead.

But, just to be clear, what I want is an accessible current carrying conductor. Is there an acceptable and safe method of providing one. These plants who want to work as safely as possible would like that capability for their folks as well. Think about the oscilloscopes with a loop passing milliamps at a voltage and frequency for calibration checking ... but with 400 amps of 575V <g>.

I do NOTHING, nor will I, in medium voltage systems (I call that 2300/4160 or higher) That's not to say that some of my hydraulic systems are not powered by electric motors there (although rare). But unless they have CTs installed and I can get to the reduced current, NOT ME!.