Twistlock and VFD

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don_resqcapt19 said:
The informational note to (a) suggests that the very fact that the motor is on a VFD is reason enough to eliminate the disconnect at the motor.
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I have never quite understood why it is not good enough to have a lockable disconnect somewhere that can be padlocked. requiring it to be "in sight" for no real reason never made sense to me.

there are so many exceptions to this rule that it would not surprise me much if there were more motors w/o a local disconnect than those with, at least in industrial settings.

a lot of plants they just contribute to unnecessary clutter out in the equipment areas and do little or nothing to improve safety.
 
petersonra said:
I have never quite understood why it is not good enough to have a lockable disconnect somewhere that can be padlocked. requiring it to be "in sight" for no real reason never made sense to me.
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Having the disconnect at the equipment make a lot of sense to me and I don't see that many exceptions to it. Depending on the AHJ there may be almost none.

Mechanical equipment is often subject to non-electrical service, inspection and maintenance by less than spectacularly trained personal.

Keeping the disconnecting means nearby and easily accessed makes it much more likely it will be used when people have to do those types of tasks.
 
petersonra said:
I have never quite understood why it is not good enough to have a lockable disconnect somewhere that can be padlocked. requiring it to be "in sight" for no real reason never made sense to me.

there are so many exceptions to this rule that it would not surprise me much if there were more motors w/o a local disconnect than those with, at least in industrial settings.

a lot of plants they just contribute to unnecessary clutter out in the equipment areas and do little or nothing to improve safety.
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There have been other threads about locating a disconnect in sight of the motor - in industrial plants. In my experience it is not common. They do little (or no) good. Maintenance is always under MCC LOTO.

The one notable exception is where there is regular cleaning required on a machine. There, in one operation I saw, the local operator opened (not locked out) a local disconnect, jump in and clean, jump out, close disconnect and resume operation. Maintenance by craft techs was under MCC LOTO.

Other than that - all is LOTO at the MCC.

Note to iwire: Yes I understand that in the north east, in industrial process areas, local disconnects are common.

ice
 
iwire said:
Understood, we are subject to inspections and following the NEC. :p
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Uggg Let me get the knife out. eeeeoouuuch - got it

Okay, replying inkind:

Understood. Luckily the AHJs here can acutally read and understand the code, and apply it as written - not as they think it should be. 430.102.B.1, B.2 and following exception are perfectly clear. It says, "a or b" - not "and what ever the AHJ thinks" :thumbsdown: However, maybe there are local admendments - are there?

Even more important: The outfits that hire me are uniformly blessed with a work force that can read. :thumbsup::thumbsup:

ice
 
iwire said:
Understood, we are subject to inspections and following the NEC. :p
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We also follow the NEC and we rarely install a local disconnect at a motor in an industrial occupancy. (b) of the exception applies to most industrial occupancies.

About the only time we install one at a motor is where that motor needs to be frequently locked out for cleaning or other normal operational purpose. The local disconnect is there only to save time for the operators in those cases.
 
So, is it only the north east where, in industrial settings, local disconnects are common?

What say y'all? Yes, No, what area.

No this is not a poll - just minor curiousity.

ice
 
iwire said:
Safety rules are to protect people, not equipment.
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Precisely. That was my point when I said "probably" wasn't good enough.

iwire said:
I see no hazard to people assuming a properly rated receptacle and plug.
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Properly rated for what? It might be reasonable to assume that it's adequately rated to carry the current.
But do you think it's reasonable to assume that it's rated to break the current?
And what if it isn't? Would you risk it?
 
Besoeker said:
Precisely. That was my point when I said "probably" wasn't good enough.


Properly rated for what? It might be reasonable to assume that it's adequately rated to carry the current.
But do you think it's reasonable to assume that it's rated to break the current?
And what if it isn't? Would you risk it?
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A lot of such plugs are HP rated here in the states.

I think you can make a very credible argument that it is mostly safe to do this. It might not be safe for either humans or the machine now and then, so I am unwilling to make a blanket statement that it is, but it probably is safe most of the time.

The cases where it might not be safe, you lock it out first.
 
petersonra said:
A lot of such plugs are HP rated here in the states.

I think you can make a very credible argument that it is mostly safe to do this. It might not be safe for either humans or the machine now and then, so I am unwilling to make a blanket statement that it is, but it probably is safe most of the time.

The cases where it might not be safe, you lock it out first.
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OK.
How does the person doing the unplugging know if it is safe to do so?
 
petersonra said:
Many places there are written instructions on such things.
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OK.
All places should have written instructions on such things.
Would the written instructions really allow live disconnection for this application?
I suspect not.
So how would you ensure it can't be a live disconnection?
 
In my experience, if you create a situation where Bubbah can grab a cord and pull it out when he is not supposed to, Bubbah will. If you train Bubbah not to, then Smitty will. You are setting this up for Murphy's Law to prevail. :happysad:

Also in my DIRECT experience with plug in connections and VFDs, the VFDs can strike and maintain an arc longer than standard AC, because remember, the output is actually DC pulses. SOME VFDs will detect the voltage rise of the arc and clamp or turn off the transistors, but some will not. It is not always part of the designer's thought process that anyone would think of doing something like that. If you ask any VFD mfr if this is OK, they will say No.

There are plenty of alternatives if you want a quick disconnect, but they usually involve pin and sleeve connectors, not twist lock. In that format, you can get them in a box where they are interlocked mechanically with a disconnect, so you cannot remove the plug unless the disconnect is open. I HIGHLY recommend that. There is also one pin and sleeve mfr who has theirs rated as disconnect switches by themselves(Meltric), so again, it disconnects power BEFORE you can remove the plug. Either of those is still not necessarily good for the drive, but they are safer for the humans. If you want to avoid stressing the drive, add an aux contact to the disconnect switch to turn the drive off before the main contacts open, then nothing is at risk.

By the way, unshielded flexible cord like SO cable on the output of a VFD? Really bad news. The output becomes a fairly powerful local radio transmitter. If you are going to do this, you should be using shielded cable, and ground that shield on BOTH ends.
 
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Jraef said:
... There is also one pin and sleeve mfr who has theirs rated as disconnect switches by themselves(Meltric), so again, it disconnects power BEFORE you can remove the plug. Either of those is still not necessarily good for the drive, but they are safer for the humans. If you want to avoid stressing the drive, add an aux contact to the disconnect switch to turn the drive off before the main contacts open, then nothing is at risk.

By the way, unshielded flexible cord like SO cable on the output of a VFD? Really bad news. The output becomes a fairly powerful local radio transmitter. If you are going to do this, you should be using shielded cable, and ground that shield on BOTH ends.
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The Meltric plugs have an option for a first break set of contacts for the control circuit and they can also be used with Flexible Metal Conduit or Liquid Tight Flexible Metal Conduit to provide some shielding.
 
I can't get past:
"So, they are not doing this for 400.7.A.6 or 400.7.A.8?"

If so, flexible cord is out. Seroiusly, if they want a disconnect out by the motor, then ..........

put in a disconnect:slaphead:

ice
 
iceworm said:
I can't get past:
"So, they are not doing this for 400.7.A.6 or 400.7.A.8?"

If so, flexible cord is out. Seroiusly, if they want a disconnect out by the motor, then ..........

put in a disconnect:slaphead:

ice
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Or use the described Meltric or other pin-and-sleeve connector that can be used with FMC or LFMC instead of cord?
 
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