Debate over turning off power

[iwire]

I don't think it is ever a good idea to diconnect or re-connect under load. Some times it can't be helped but I always find these situations to be scary.

That is odd, the NEC allows disconnect switches and breakers to be the controller for a motor. Are you suggesting the NEC is allowing 'scary' installations?

See 430.83(A)(1) and (2).

No matter where you open the circuit it is done under load, regardless of it being a switch, contactor or relay.

 

[iwire]

Even though breakers and disconnects may be designed to "handle" opening under load, it is a good operational practice not to force them to do so.

No matter how we turn equipment on or off some device is doing so under load and is being used up.

 

[infinity]

It is my understanding that a disconnect is rated to switch under full load and a safety switch is not rated to switch under full load only to lockout power after the feeder has been deenergized. That is what I was taught.

So do you mean that switches are listed to be either a disconnect or safety switch? I've never heard of that distinction and would be surprised to learn that a distinction exists. If so, wouldn't all safety switches require some sort of warning labels to say that they cannot be opened or closed under load?

 

[iwire]

So do you mean that switches are listed to be either a disconnect or safety switch?

I think quogueelectric is mistaken.

Square D calls them all 'Safety Switches' and as long as the switch is rated in HP it can be used as the controller per 430.83(A)(1).

 

[weressl]

Disconnects are designed to open the circuit under load. Many of the larger ones have arc chutes and arc suppresion methods built into them. Discos are also quick-make/quick-break to minimize arcing.

Breaker or disco, if there's a load, there's an arc. Both are designed to handle it.

Be careful there. Some disconnects are designed to interrut load and some are not.

 

[480sparky]

Be careful there. Some disconnects are designed to interrut load and some are not.

So how to tell the difference?

 

[weressl]

So how to tell the difference?

Knowing your equipment should be a pre-requisite for a qualified practicing electrician. As we can see from the majority of the 'answers' it is not so as people seem to be unavare that such distinction as disconnect and load-break disconnect are two different animals. Manufacturers instructions are the first line of defense, but on existing equipment it is mostly useless. Sometimes the switches are labeled so. In the case of MV equipment it is a much clearer issue. Ex. the disconnect on an AMPGARD MV starter is not suitable for load interruption and has an interlock that supposed to prevent it, yet an arc-flash incident I am aware of testifies that with enough muscle power that can be broken.

 

[iaov]

That is odd, the NEC allows disconnect switches and breakers to be the controller for a motor. Are you suggesting the NEC is allowing 'scary' installations?

See 430.83(A)(1) and (2).

No matter where you open the circuit it is done under load, regardless of it being a switch, contactor or relay.

I wouldn't say the NEC is allowing scary stuff. I'm saying that over the years I've seen enough stuff blow up that I would much rather have a remote contactor doing the disconnecting/reconnecting if possible.

 

[LarryFine]

I'm saying that over the years I've seen enough stuff blow up that I would much rather have a remote contactor doing the disconnecting/reconnecting if possible.

Or a helper.

 

[quogueelectric]

It must be a full moon tonight I have had this response all night.

 

[LawnGuyLandSparky]

It must be a full moon tonight I have had this response all night.

It's just amazingly QUIET tonight on the Island, like a calm before a storm, isn't it?

 

[wdemos]

I agree with Laszlo 100% and I feel using a circuit breaker or any switch which may or may not be a load rated disconnect to shut down any piece of equipment makes no sense to me if there is a properly sized starter to handle such a simple task. And then do your LOTO as required.

 

[George Stolz]

Knowing your equipment should be a pre-requisite for a qualified practicing electrician. As we can see from the majority of the 'answers' it is not so as people seem to be unavare that such distinction as disconnect and load-break disconnect are two different animals. Manufacturers instructions are the first line of defense, but on existing equipment it is mostly useless. Sometimes the switches are labeled so. In the case of MV equipment it is a much clearer issue. Ex. the disconnect on an AMPGARD MV starter is not suitable for load interruption and has an interlock that supposed to prevent it, yet an arc-flash incident I am aware of testifies that with enough muscle power that can be broken.

Laszlo, most of us don't work on MV equipment, and therefore that type of equipment really shouldn't be brought into a general discussion, especially as a tool to confuse and belittle people.

I will also offer this section in addition to the ones already offered,to bolster the idea that you can disconnect a load from any point.

110.9 Interrupting Rating. Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment.
Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.

I too would use the factory controls to shut something down before disconnecting it, but at the end of the day all switches installed have to be able to handle the voltage and current involved.

 

[JWCELECTRIC]

It's not recommended to turn motor off with disconnect if there is a VFD for a starter, could damage the motor and the VFD. Either use the controls, or HOA switch so the VFD ramps down. Use the disconnect switch as a last resort if there is an emergency.

 

[weressl]

Laszlo, most of us don't work on MV equipment, and therefore that type of equipment really shouldn't be brought into a general discussion, especially as a tool to confuse and belittle people.

Is MV part of the NEC or not?
MV was bought up as an EXAMPLE in ADDITION to LV equipemnt as one where this - disconnect switch v.s. load-break disconenct - distinction is clearer. I stand by my statement that a practicing electrician SHOULD know the difference. I placed the fault also on the manufacturers and the listing agencies who does not label and require labeling accordingly. Having global experience in the electrical business the required knowledge in the US is subpar. This is my experience and opinion. Please allow me to have it.

 

[bjdm829@fuse.net]

right on Charlie, always disconnect at the load first, (control circuit, lights,etc)then go to the expensive equipment, unless u hope to be hired at a latter date for replacement.

 

[don_resqcapt19]

430.109(E) Isolating Switches. For stationary motors rated at more than 40 hp dc or 100 hp ac, the disconnecting means shall be permitted to be a general-use or isolating switch where plainly marked “Do not operate under load.”

 

[Hendrix]

And some of those people go on to be inspectors. Or should I say expectors.

But most don't

 

[shockin]

I wonder if this is just a simple misunderstanding. Maybe the OP didn't fully understand what his co-worker was telling him. You could interpret it this way instead:

In order to prevent / reduce the risk of arc flash one must first disconect power at the source. (breaker) then go to the disconnet at the machine and confirm power is disconnected. The LOTO must be applied at the breaker because simplying turning off the disconnect at the machine will not eliminate all live parts with-in the machine as required by NFPA 70e.

Just a thought - there might be more to the story.

 

[pfalcon]

I heard from this guy that he was told that your suppose to turn off the 'breaker' that feeds the disconnect before turning off the disconnect that feeds a 'motor' or ac unit when working on the unit/motor.

I always was taught it did not matter so long as you secured the power to the equipment you were working on (and of course test to be sure its off).

He claims he was told to do it that way because the 'disconnect' has a greater chance of 'arc flash'.

I have heard of disco's blowing a cover but so can a 'breaker' there is still an 'arc' to some degree. Anyone ever hear of this practice??

A) Shut down the equipment normally.
B) Unless the breaker is rated as a switch or is the lockout point; do not turn it off.
C) Use safe working practices to turn off the disconnect and lock it out.

Think about it this way: A breaker is made to interrupt a fault; A disconnect is made to remove power for service. Use the two devices for the purpose they were made. Don't try to compensate for urban legends.