Small Motor Incident

[reynokj]

An air conditioning contractor was using a vacuum pump and received a shock when he touched the hose connection port of the pump. The motor for this pump was European Style (220 Volt to Neutral) and the power source was US style (120 volt to Neutral, 240 volt between phases). After investigating this incident, I found that the vacuum pump was connected to a 220 volt source using a 2 pin adapter, thus eliminating the grounding conductor, this being the primary cause of the incident. Upon further investigation the following items were noticed concerning this motor:

1. Motor "On/Off" Switch was a single pole type
2. Motor windings were megged and checked out good
3. Windings resistance was very good
4. A voltage of 110 Volts was measured between the case and ground without grounding conductor.
5. Injured person was sweating profusely when the incident occurred
6. Vacuum Pump had rubber posts underneath to absorb vibration
   
   My question is not what caused this incident, but how we would rate the possible severity and what could be the possible cause of this high of a voltage occurring on this motor case, even in the "off" position with seemingly good winding insulation? What type of strength could this "Field" have concerning the severity?
   :-?
   

 

[Volta]

Welcome to the forum.

I am not familiar with the European style, is it three-wire (hot, neutral, grounding)?

Thus the two pin adapter was to connect the current-carrying conductors but not the body/grounding conductor? Or was it to adapt from a NEMA 6-xx to the European style?

You say the "Windings resistance was very good", is that to mean the the megger test showed a "very good" resistance from the body to each of the current-carrying conductors (hot or neutral)? With the switch closed?

You also say that your "question is not what caused this incident, but . . . and what could be the possible cause of this high of a voltage occurring on this motor case". That is the same question, IMO. The switch was closed when the shock occured, so either conductor may have been shorted to the case, and possibly only with a certain tension on the cord or similar that didn't occur during your test.

 

[dbuckley]

The megger is measuring DC resistance from windings to case.

I suspect there is a capacitive connection, and thus when line supplied, without a ground connection, the case becomes live, albeit through capacitive impedence.

The root cause here is "user is idiot" (I've made that more polite than I would in face to face communication) - dont not connect grounds, thats how bad stuff happens.

 

[reynokj]

The megger is measuring DC resistance from windings to case.

I suspect there is a capacitive connection, and thus when line supplied, without a ground connection, the case becomes live, albeit through capacitive impedence.

The root cause here is "user is idiot" (I've made that more polite than I would in face to face communication) - dont not connect grounds, thats how bad stuff happens.

Your last statement is correct, but not necessarily for the user but for the electrician that supplied power to the Factory Service Rep. I totally agree with the capacitive effects of the motor. The motor being built to European standards assumes 220 volt to Neutral and a ground (3 Conductors) our vessel is very similar to the 240 volts that we see in the US as 120 volt Line to Neutral and 240 volts line to line. The single pole switch was allowing the windings to be energized at all times with potential, but the circuit would not be complete until the switch was closed and the motor started. The case to ground voltage would actually decrease to approx 69 volts when the motor was running, but return to 110 volts when the switch was opened and the motor stopped. Testing of the windings were perfect, no signs of leakage to case when tested at 250 volt DC. Also, tested the resistance of the windings themselves with no abnormal properties. The start Capacitor was removed from the circuit to eliminate any chance of leakage during the test, and there was no change in the case to ground voltage. The discharge of this voltage was so small that the meters that I had were not picking it up. Only a minor "blip" in current was evident on my old (but trustworthy) Simpson 260 when discharged. I may not have been clear in my original message, but I am trying to determine how much energy this "capacitive" charge could build up. Could it cause death or just a "get you attention" type of incident. I have been asked to rate the potential seriousness of "this" incident. The potential for not connecting the ground is "through the roof" but I need to class this particular case.

Thanks for you help!! Sorry I was so late returning to the site, but being a new member, I had a little navigating the site and trouble finding where my thread was placed!!!

 

[reynokj]

Welcome to the forum.

I am not familiar with the European style, is it three-wire (hot, neutral, grounding)?

Thus the two pin adapter was to connect the current-carrying conductors but not the body/grounding conductor? Or was it to adapt from a NEMA 6-xx to the European style?

You say the "Windings resistance was very good", is that to mean the the megger test showed a "very good" resistance from the body to each of the current-carrying conductors (hot or neutral)? With the switch closed?

You also say that your "question is not what caused this incident, but . . . and what could be the possible cause of this high of a voltage occurring on this motor case". That is the same question, IMO. The switch was closed when the shock occured, so either conductor may have been shorted to the case, and possibly only with a certain tension on the cord or similar that didn't occur during your test.

Yes, European and Asian Countries use a 220 volt line to Neutral with a ground, not the US style of 240 volt line to line and 120 line to neutral. The motor windings were tested first by simply testing from the plug to the case and everything was good according to facility electrician. I brought the motor into the lab and separated all of the winding leads and tested individually, DC megger settings were 250 Volt. No indication of shorting in the windings were evident. I then tested the internal resistance of each winding with no signs of abnormal resistance. The single pole switch that I mentioned in the previous thread only "opened" one side of the 220 volts, therefore, the windings stayed energized with 110 volts potential, albeit no closed circuit therefore the motor wasn't running until the switch is closed. With start capacitor removed from the circuit, the "field" was still present on the case of the motor, even with the switch off. The field weakened to about 69 volts when the motor was running. I also tested a new Vacuum pump without the ground and this field strength was 28 volts to ground.

The primary cause of this incident is the failure of the electrician to properly ground the Vacuum Pump before allowing the Factory Service Representative to use it. I am trying to rate this particular incident. Could the development of this "field" or "capacitive effect" been of sufficient potential to cause serious injury or death, or should it be more assimilated to a static charge? We realize the seriousness of the elimination of the grounding conductor and have rated that as "extreme". What my "management" wants to know is what could the "charge" in this particular incident have caused. Yes, I realize this type of ranking is ridiculous and not to the point of the actual incident, but just trying to do my "due diligence" and cover all aspects.

Thanks very much for your help and sorry for the late return of a "reply". I am new to the site and had trouble locating my Thread!!!