Upper Management

[Mister Rogers]

Hey everybody, just wanted to ask for your thoughts on a work situation.
During a recent emergency at work, a member of upper management at our facility asked 4 of my electricians to hook up a 480 volt 6KW portable electric heater to the motor junction box(aka peckerhead) of one of the station dry pit submersible pumps. :happysad:
The pump motor is 3 phase/460/135hp/101kw. There are 4 of them in the station. Each is controlled by its own VFD in the MCC on the upper level. The motors are rated class 1 div 1 and are located in the drywell of the station. Each is fed by a parallel pair of extra hard usage SO cable, made by the manufacturer.
The station had been flooded and at this point he wanted to get the portable heater energized to start drying out the moisture in the motors and their cables. Meantime, a contractor was still working in the room, pressure washing the sludge off of the walls.
my guys said "no", it's a code violation and very unsafe. The next day I had a pretty charged conversation with him explaining how wrong he was to ask them to do that, they weren't being insubordinate or incompetent, on the other hand they were being professional. I said I would have told him the exact same thing if he asked me to do that.
Now...he has challenged me to explain what NEC code violations could there have been if the guys did what he told them to do.

So my question to you is just the same, what code articles would you say would have a bearing on my boss's boss's boss's installation of a portable heater cable in the peckerhead of a hazardous location motor which is fed from a VFD? LOL

here is what I have come up with so far:

110.12
110.27
110.31(B)(1)
210.20A
240.5(B)(1)
240.21
424.93
this only took about 15 mins, I haven't even really started with 430 yet
430.62A
430.24

thanks guys,

Mr. Rogers

 

[GoldDigger]

... which is fed from a VFD? ....

So the VFD would be driving the heater with the motor disconnected or just driving the motor and the heater in parallel?

Either one sounds really wrong to me unless the VFD was put into bypass mode.
I guess how well it would work would depend in part on the VFD settings, but I would also worry about the high dV/dt of the VFD output damaging the heater.

 

[iwire]

this only took about 15 mins, I haven't even really started with 430 yet

My only suggestion is not to make a long list of violations, one maybe two of clear cut, no gray area sections and that is it.

One violation is a violation, in my opinion presenting a long list them just seems like your not sure of yourself.

I would stick with the basics, where was the OCPD for this heater? Was it sized correctly?

Is the output of the VFD supplying the heater consistent with the heaters labeling? Volts, hz etc.

 

[Smart $]

This one comes first and foremost in any list.

ARTICLE 590
Temporary Installations
590.1 Scope. The provisions of this article apply to temporary
electric power and lighting installations.

590.2 All Wiring Installations.

(A) Other Articles. Except as specifically modified in this
article, all other requirements of this Code for permanent
wiring shall apply to temporary wiring installations.

(B) Approval. Temporary wiring methods shall be acceptable
only if approved based on the conditions of use and
any special requirements of the temporary installation.

 

[Saturn_Europa]

I would present one or two clear violations of the NEC. Explain how these violations can expose employees and contractors to unsafe working conditions. Then provide a solution to the problem. Good luck!!

 

[zbang]

I would present one or two clear violations of the NEC. Explain how these violations can expose employees and contractors to unsafe working conditions.

Definitely, but keep a few more in reserve. How about "These are the most important/unsafe violations, but there are others that would also need to be addressed."?

 

[Mister Rogers]

Thanks

Thanks

Thanks guys, this really helps.
GoldDigger, yes, he wanted the pump and the motor hooked up in parallel, both running on the VFD.

I like the idea to keep it simple, I'm going to do that.

 

[Saturn_Europa]

GoldDigger, yes, he wanted the pump and the motor hooked up in parallel, both running on the VFD.

That's crazy

 

[kwired]

If you were to temporarily connect the heater to the motor circuit and either use a heater rated for the classified area or maybe even provide sufficient ventilation to temporarily de-classify the area it may be alright.

 

[SceneryDriver]

Thanks guys, this really helps.
GoldDigger, yes, he wanted the pump and the motor hooked up in parallel, both running on the VFD.

I like the idea to keep it simple, I'm going to do that.

Especially if the heater was single phase, the VFD would almost certainly trip on phase current unbalance. If it was three phase, you would most likely have to set the VFD motor overload protection to an unsafe high level to get both to run without tripping on overcurrent. It would most likely not work.

What was the supposed goal of using the heater, anyway? To dry out the peckerhead? Probably a bad idea to try to run the motor you're trying to dry out. :blink:


SceneryDriver

This has "stupid idea" written all over it. You and your guys were right to refuse.


SceneryDriver

 

[kwired]

I also have to question if a VFD will like what it sees if you connected a resistance load to it?

Changes in frequency will not change the output current or power, it may think there is a problem if response to such changes don't yield expected changes.

I know you don't put a capacitor in the output, but not sure what resistance loads will do.

 

[Jraef]

I also have to question if a VFD will like what it sees if you connected a resistance load to it?

Changes in frequency will not change the output current or power, it may think there is a problem if response to such changes don't yield expected changes.

I know you don't put a capacitor in the output, but not sure what resistance loads will do.

A VFD can run a heater, OR it can run a motor. Can it run a heater AND a motor at the same time? Probably yes, but it would not do well with either of them.

When a VFD is connected to a 3 phase heater, the resistance is fixed so kwired is right, the VFD will not react the same as it does for a motor, but it's OK to do that, there is just no point on a regular basis, you gain nothing by using a VFD on that heater compared to a much less expensive SCR controller. But in an emergency? Sure. Side note: the VFD would need to be in V/Hz mode, because any kind of vector mode will not work.

When it is connected to BOTH, the problem will be that the added resistance of the heater in parallel with the motor impedance will lower the total circuit resistance that the VFD sees (think resistors in parallel). So assuming someone is smart enough to set the Current Limit function of the VFD so that it doesn't trip out, that will mean the VFD will have to artificially lower the output frequency to try to keep the current under control. That then will affect the motor's ability to perform, possibly to the point of it stalling. We are also ASSuming this is a purely resistive heater, which for the purposes of drying something out, would be next to useless. So more likely this is a unit heater with a blower. The artificially reduced frequency going to the blower motor (assuming it is 3 phase as well) will mean that the blower will NOT move as much air, and if the heater is DESIGNED for that air flow, the heating coils may burn out. If the blower motor is single phase, fuggetaboudit... Get out the marshmallows, it's going to be a campfire.

So code wise, I'm in the 110.3 (B) camp: suitable for the use.

(B) Installation and Use. Listed or labeled equipment
shall be installed and used in accordance with any instructions
included in the listing or labeling.

I'm almost certain that you will not find anything in the instructions for the heater, motor or VFD saying that ANY of this is OK. You really should not have to go further than that.

You MAY on the other hand find info on the VFD stating that it can be used for resistance heater control alone. So if he wants to dry out the pit, he can in theory do that by NOT connecting the motor at the same time, but the blower motor issue would still be there.

 

[kwired]

A VFD can run a heater, OR it can run a motor. Can it run a heater AND a motor at the same time? Probably yes, but it would not do well with either of them.

When a VFD is connected to a 3 phase heater, the resistance is fixed so kwired is right, the VFD will not react the same as it does for a motor, but it's OK to do that, there is just no point on a regular basis, you gain nothing by using a VFD on that heater compared to a much less expensive SCR controller. But in an emergency? Sure. Side note: the VFD would need to be in V/Hz mode, because any kind of vector mode will not work.

When it is connected to BOTH, the problem will be that the added resistance of the heater in parallel with the motor impedance will lower the total circuit resistance that the VFD sees (think resistors in parallel). So assuming someone is smart enough to set the Current Limit function of the VFD so that it doesn't trip out, that will mean the VFD will have to artificially lower the output frequency to try to keep the current under control. That then will affect the motor's ability to perform, possibly to the point of it stalling. We are also ASSuming this is a purely resistive heater, which for the purposes of drying something out, would be next to useless. So more likely this is a unit heater with a blower. The artificially reduced frequency going to the blower motor (assuming it is 3 phase as well) will mean that the blower will NOT move as much air, and if the heater is DESIGNED for that air flow, the heating coils may burn out. If the blower motor is single phase, fuggetaboudit... Get out the marshmallows, it's going to be a campfire.

So code wise, I'm in the 110.3 (B) camp: suitable for the use.


I'm almost certain that you will not find anything in the instructions for the heater, motor or VFD saying that ANY of this is OK. You really should not have to go further than that.

You MAY on the other hand find info on the VFD stating that it can be used for resistance heater control alone. So if he wants to dry out the pit, he can in theory do that by NOT connecting the motor at the same time, but the blower motor issue would still be there.

Most unit heaters with a single phase motor are likely PSC or shaded pole motors - I would think they would be fine as well - but still have the problem of not moving enough air to keep the heater coils cool enough.

 

[wyreman]

we had a huge generator big as a tugboat
that had a "battle mode"
that would let you run it under any circumstances until it would utterly fail

what was your "emergency"
life and death or

a little production slowdown?

 

[petersonra]

It does not seem like it is especially unsafe to me. Maybe not the best solution to the problem but not particularly unsafe either. I'm sure we could all find a code violation and was being suggested but I don't think we could find a clear-cut case of something being truly unsafe

 

[iwire]

It does not seem like it is especially unsafe to me. Maybe not the best solution to the problem but not particularly unsafe either. I'm sure we could all find a code violation and was being suggested but I don't think we could find a clear-cut case of something being truly unsafe

I don't get your point in the least.

The OP asked for code violations, not random people's feelings of safety.

What would your reaction be to your upper management asking you to do things that could jeopardize your personal credentials?

 

[petersonra]

I don't get your point in the least.

The OP asked for code violations, not random people's feelings of safety.

What would your reaction be to your upper management asking you to do things that could jeopardize your personal credentials?

The OP referred to the situation as "very unsafe".

 

[wtucker]

what code articles would you say would have a bearing on my boss's boss's boss's installation of a portable heater cable in the peckerhead of a hazardous location motor which is fed from a VFD?

Why bother with the NEC? It's OSHA that's the issue! https://www.osha.gov/right-to-refuse.html