Help with wiring issue

[Daymeone]

Hi, I'm new to this forum and need advice on a issue I had with an industrial mixer.
One of the kitchen staff got shocked when they touched a screw on the housing of the mixer. The mixer has 230v feed and no neutral wire. The ground wire was disconnected. When I reconnected the ground wire the problem went away. My question is did the motor induce electricity on the housing screw? Is there a more Code compliant way to resolve this issue?

 

[GoldDigger]

Hi, I'm new to this forum and need advice on a issue I had with an industrial mixer.
One of the kitchen staff got shocked when they touched a screw on the housing of the mixer. The mixer has 230v feed and no neutral wire. The ground wire was disconnected. When I reconnected the ground wire the problem went away. My question is did the motor induce electricity on the housing screw? Is there a more Code compliant way to resolve this issue?

Induced is probably not the best word, since it implies induction as the mechanism.
Chances are that the screw is connected to an internal metal part that is getting either capacitive leakage current or resistive leakage because of dirt or failed/failing insulation.
The current is probably small enough that grounding the metal to provide a low resistance return current path is all that is needed. But I would measure the current anyway to confirm as it may be a sign of a bigger problem developing.
Equipment designed to operate without an EGC connection (called double insulated) is rigorously designed so that no single insulation failure can energize exposed parts. Incorrect repair of double insulated equipment can cause a hazardous condition.

 

[JFletcher]

Welcome to the forum!

Hi, I'm new to this forum and need advice on a issue I had with an industrial mixer.
One of the kitchen staff got shocked when they touched a screw on the housing of the mixer. The mixer has 230v feed and no neutral wire. The ground wire was disconnected. When I reconnected the ground wire the problem went away. My question is did the motor induce electricity on the housing screw? Is there a more Code compliant way to resolve this issue?

Are you talking about something like a Hobart dough mixer? If so (or even if not), get an authorized repair person out there to look at the machine. No part of the machine should cause shock to a user if it is operating correctly. Hooking up the EGC will allow a bad enough fault to trip the breaker. A lesser fault will energize the frame of the mixer to 120V (assuming just one leg/wire has faulted somewhere) - hooking up that EGC allows that voltage to go back to the source vs the user, tho if the poor dough guy has a low enough resistance, like sweaty hands touching another grounded/bonded piece of equipment, he still may get a shock with that EGC hooked up.

Get an appliance guy out there to find out what is going/went wrong inside the machine. Could be something as simple as insulation rubbed thin or off on a wire that is touching the metal frame.

Eta: I have used several of the aforementioned Hobart dough mixers and it's routine to touch the screw and bowl or frame simultaneously with wet hands. If I had gotten shocked by that, it would have been the last batch of dough i would have made until it was fixed properly.

 

[ramsy]

..If I had gotten shocked by that, it would have been the last batch of dough i would have made until it was fixed properly.

Roger that, Kitchen GFCI's will disconnect most appliances leaking current to ground.

If 2017 NFPA-70 applies to your region, 210.8(B) now clarifies GFCI's are required for any outlet in commercial kitchens 150v to ground or less. That includes 230v appliances with no neutral, up to 50 amps.

 

[domnic]

egc

egc

Induced is probably not the best word, since it implies induction as the mechanism.
Chances are that the screw is connected to an internal metal part that is getting either capacitive leakage current or resistive leakage because of dirt or failed/failing insulation.
But I would measure the current anyway to confirm as it may be a sign of a bigger problem developing.
Equipment designed to operate without an EGC connection (called double insulated) is rigorously designed so that no single insulation failure can energize exposed parts. Incorrect repair of double insulated equipment can cause a hazardous condition.

The current is probably small enough that grounding the metal to provide a low resistance return current path is all that is needed. ? please explain. the egc is installed to clear the fault . if it does not it is worthless.

 

[cowboyjwc]

The current is probably small enough that grounding the metal to provide a low resistance return current path is all that is needed. ? please explain. the egc is installed to clear the fault . if it does not it is worthless.

replace the word grounding with bonding.

 

[RumRunner]

Missing text

Missing text

Hi, I'm new to this forum and need advice on a issue I had with an industrial mixer.
One of the kitchen staff got shocked when they touched a screw on the housing of the mixer. The mixer has 230v feed and no neutral wire. The ground wire was disconnected. When I reconnected the ground wire the problem went away. My question is did the motor induce electricity on the housing screw? Is there a more Code compliant way to resolve this issue?

Most industrial mixers are twistlock cord and plug connected. The bowl in some localities require them to be removable for disinfection or just thorough cleaning.
These machines are mostly resting on rubber pads because OSHA requires them to minimize "walking".
What I can't figure is why does the operator gets shocked when touching the screw. . .is the bowl made of rubber or glass? The disconnected ground wire and reconnected could be just what it needs.
Well OK, that doesn't explain why there is stray voltage.:blink:

 

[ptonsparky]

Repairing the EG is needed, but that does not negate the fact that you have a problem. The reattached EG will allow current to flow back to the source while lowering the touch potential of the mixer. Generally there should be no current flow on the EG of a properly operating piece of equipment.

 

[domnic]

mlxer

mlxer

Repairing the EG is needed, but that does not negate the fact that you have a problem. The reattached EG will allow current to flow back to the source while lowering the touch potential of the mixer. Generally there should be no current flow on the EG of a properly operating piece of equipment.

NOT TRUE . your in parallel with the voltage.

 

[ptonsparky]

NOT TRUE . your in parallel with the voltage.

Explain your thoughts.

 

[winnie]

If there is a high impedance fault in the mixer, meaning a fault that permits _limited_ current to flow to the frame of the mixer, then with the EGC disconnected the frame will become energized. Someone touching the frame could feel a shock.

If this fault is not corrected, but the EGC is connected, then this current will flow through the EGC back to the neutral bond at the service panel, and from there back to the transformer. This will _reduce_ but not eliminate any touch potential on the frame of the mixer.

Someone touching the frame of the mixer would be placed in parallel with the EGC path, and _might_ still experience a shock. But a good EGC path will probably eliminate the shock; with a high impedance fault and a good EGC the touch potential will likely be reduced to the point that no shock is experienced.

With a _low impedance_ fault, a shock from the frame is quite likely while the circuit is energized, but because lots of current is flowing the breaker will likely trip quickly, eliminating the shock hazard. The shock hazard exists because of the voltage drop in the EGC with the high current flow.

In a mixer with no fault at all, if the EGC is not connected, there is a good chance that someone touching the frame will experience a minor shock due to the _expected_ leakage current. Capacitive coupling from the motor windings to the frame or minor and acceptable leakage across insulation can energize the frame. This current should be very low, in the microamp range. An EGC will remove this shock hazard.

In a system with moving parts you also have the possibility of electrostatic charging with no fault or leakage in the electrical system. I'd think this is unlikely with wet dough...but a possibility to consider especially if the mixer is used first on dry ingredients. An EGC will also act to drain this static charge.

-Jon

 

[JFletcher]

If there is a high impedance fault in the mixer, meaning a fault that permits _limited_ current to flow to the frame of the mixer, then with the EGC disconnected the frame will become energized. Someone touching the frame could feel a shock.

If this fault is not corrected, but the EGC is connected, then this current will flow through the EGC back to the neutral bond at the service panel, and from there back to the transformer. This will _reduce_ but not eliminate any touch potential on the frame of the mixer.

Someone touching the frame of the mixer would be placed in parallel with the EGC path, and _might_ still experience a shock. But a good EGC path will probably eliminate the shock; with a high impedance fault and a good EGC the touch potential will likely be reduced to the point that no shock is experienced.

With a _low impedance_ fault, a shock from the frame is quite likely while the circuit is energized, but because lots of current is flowing the breaker will likely trip quickly, eliminating the shock hazard. The shock hazard exists because of the voltage drop in the EGC with the high current flow.

In a mixer with no fault at all, if the EGC is not connected, there is a good chance that someone touching the frame will experience a minor shock due to the _expected_ leakage current. Capacitive coupling from the motor windings to the frame or minor and acceptable leakage across insulation can energize the frame. This current should be very low, in the microamp range. An EGC will remove this shock hazard.

In a system with moving parts you also have the possibility of electrostatic charging with no fault or leakage in the electrical system. I'd think this is unlikely with wet dough...but a possibility to consider especially if the mixer is used first on dry ingredients. An EGC will also act to drain this static charge.

-Jon

Another possibility, though unlikely, is that the worker had built up a static charge on him, and discharged it when touching the mixer. A properly functioning mixer connected to an EGC or even GFCI will not prevent that type of shock. We dont know if the worker got a static charge zap or a few ma from a 120V fault.