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Residual charge in 4 kV motor - any limits?

Residual charge in 4 kV motor - any limits?

Residual charge in 4 kV motor - any limits?

(OP)
The phone rang just before midnight. That is usually a bad omen - someone needing immediate help with some critical equipment. I am all for THAT kind of problems. But this was different.

One of the electrical guys was replacing a 4 kV motor, probably around a few MW size. The motor was disconnected (locked out and tagged). He wanted to check insulation and, doing this, he touched one of the terminals. He felt a sharp discharge from terminal to finger. He didn't really understand why - no hipot had yet been connected.

This is what the phone call was about. My understanding is that the insulation was in perfect order and that there was some charge left in the winding when the motor was switched off and he discharged it when touching the terminal. Perfectly normal and nothing to worry about. I would perhaps discharge the winding before touching it. But this guy didn't.

Then, someone pressed the BIG RED ALARM button and this little incident soon became a VERY IMPORTANT ELECTRICAL ACCIDENT with all safety officers and MBA:s involved. I just laughed and said that they could go back to sleep. I certainly would do just that.

In the morning (right now) I made a rough calculation to find out how dangerous, if at all, that spark could be. Assuming a maximum capacitance Winding-Frame to be around 60 nF and peak voltage 4000/(sqrt(3))*sqrt(2) I arrived at an energy contents being in the order of 300 mJ.

Electric fences can deliver up to 20 J at 10+ kV levels. And I have yet to see anyone hurt (other than a certain not-so-pleasant feeling) from that. So, I do not think that a few percent of the energy at (at most 30 percent) of the voltage that you find in fences around fields in most of the world merits a full-blown panic investigation.

But that is me - what do you think?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Residual charge in 4 kV motor - any limits?

I generally agree with your assessment. I like the fence comparison. Even an MBA should be able to relate those two.

I can see the management types being concerned about staff even being annoyingly shocked and how it could mean procedurally something should be improved to prevent annoying shocks, because there is only a small separation between annoyance and fatal.

Example: would be if the guy had first checked the cables for being energized he'd have actually seen the potential and not gotten shocked.
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Quote:

Electric fences can deliver ......... And I have yet to see anyone hurt

So.

Clearly.

You've never peed on an electric fence.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Residual charge in 4 kV motor - any limits?

(OP)
My neighbours do that regularly. It adds a certain je ne sais quoi to their xxx life. They say.


You are right - I never tried it. But is it lethal? Really, like in driving an MC into a church wall? They do that, too.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Residual charge in 4 kV motor - any limits?

Peeing on an electric fence! What an interesting alternative to a full on Darwin award.
The motor becomes a capacitor. Treat it as a capacitor without a bleed resistor.
Discharge before touching.
A voltage test may be misleading.
Non-contact voltage testers (Tic-Testers) have become ubiquitous in the field.
Tic-Testers don't work well on DC.
Discharge a high voltage motor whether it needs it or not.
Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Residual charge in 4 kV motor - any limits?

The actual jolt he received probably wasn't all that harmful, but at say 11kV it would have been rather more energy involved.

The management should be concerned about a safe system of work which somehow allowed a worker to remove covers and access the windings of an HV machine without having first applied a circuit earth and then proven dead the machine terminals. In the UK whoever issued the permit to work would have some awkward questions to answer, not to mention likely having his switching authorisation revoked.

RE: Residual charge in 4 kV motor - any limits?

(OP)
Agree, but this was identified as a serious accident. There are classes describing the serious ness and this was classed as high as possible before actually death of the victim.

My original question was about limits - voltage or energy - and what the consequences should be. Pressing the biggest red button for an occurence like this is out of propotions. The fact that they called me in the middle of the night (I have nothing with this mill to do) is clear evidence that the panic was total.

Safety training is one thing. Sense of proportions is another.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Residual charge in 4 kV motor - any limits?

Quote (Gunnar)

Safety training is one thing. Sense of proportions is another.
I have observed that those concepts are often mutually exclusive.
There is a school of thought that holds that the reaction to a minor shock is the source of hazard rather than the shock itself.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Residual charge in 4 kV motor - any limits?

In regards to the fence picture:
Riding a bus in the third world;
There was an oncoming bus stopped at the side of the road. Our bus slowed down to see if assistance was required.
As we got closer we saw about seven men in a field beside the bus, standing in a line, all in the same position as the boys in the fence photo.
What a missed photo opportunity.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Residual charge in 4 kV motor - any limits?

Mythbusters did a show on the idea of peeing on an electric train rail. The myth was pretty much busted unless they got the test dummy within 3" of the rail. The drops separated, they even had it on high speed film.

https://www.youtube.com/watch?v=pDY-0ijiOEQ


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Residual charge in 4 kV motor - any limits?

Thanks for the link Jeff! That's got to be about the only episode I missed.

Ugh a pretty lame setup with little detail. Not one of their better ones.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Residual charge in 4 kV motor - any limits?

(OP)
A TRAIN RAIL?

Oh, I see. The supply rail on BART? They only have around 700 V DC. Not worth peeing on.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Residual charge in 4 kV motor - any limits?

(OP)
Daylight coming. If you wonder what I was doing up in the middle (almost) of our night, there is an obvious answer. And it was not about answering a phone call.

First prize: Not telling.
Second and third: Likewise.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Residual charge in 4 kV motor - any limits?

This is one of those threads where we need a 'made me laugh' button as well as the little pink stars. big smile

RE: Residual charge in 4 kV motor - any limits?

He was creeping back from his neighbor's electric fence line.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Residual charge in 4 kV motor - any limits?

Gender disorientation is not the only condition where;
"Don't ask. Don't tell.",
is an appropriate response.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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