Debate over turning off power

[brother]

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??

 

[480sparky]

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.

 

[Jim W in Tampa]

i see no hazard that would not be equal or higher at the breaker. Stand off to the side to be safe

 

[LarryFine]

Why not turn the unit off with the controls first?

 

[quogueelectric]

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??

He may be talking about the difference between a disconnect switch and a safety switch which is not designed to be switched under load

 

[infinity]

He may be talking about the difference between a disconnect switch and a safety switch which is not designed to be switched under load

What's the difference?

 

[Pierre C Belarge]

Some people are taught anothers personal preference of installations/methods and then this gets passed down as the 'code' or means of which work has to be performed.

If the equipment is installed properly, I cannot see why it makes a difference, other than personal preferences.

 

[480sparky]

Some people are taught anothers personal preference of installations/methods and then this gets passed down as the 'code' or means of which work has to be performed. ...........

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

 

[charlie b]

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. Each time such a device encounters an arc, it is diminished to some degree. Repeated operations under load will eventually result in the need to replace the device. OK, so you will tell me that they have been tested to 10,000 or more operations under load. My response is, why should we spend any amount of a component's remaining useful life, if we can avoid doing so by turning off the load first? We cannot know, when we reach for the breaker or the disconnect, what its internal condition might be. If we have the option of turning the load off or leaving it on, I submit that it is a better (and safer) habit to turn it off.

As to the original question about which is better to open first, the breaker or the local disconnect, my answer is whichever one has, through its operational lifetime, suffered the least amount of deterioration. :wink:

 

[Pierre C Belarge]

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

Hey, I resemble that remark.:grin:

 

[480sparky]

Hey, I resemble that remark.:grin:

I'd expect nothing less!:grin:

 

[hardworkingstiff]

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??

This makes no sense to me. The disconnecting means if properlly installed can handle the "break" and "make". As far as 'arc flash', there is more potential at the breaker than there is at the disconnect.

Why is it safer to turn off a breaker under load than a disconnect under load? If there is no load, then there is no problem.

 

[brother]

I thought this was a far stretch of doing these things, must have been a personal preferrence of someone. It just didnt make any sense to me about this issue of the disconnect seeing they were made for that purpose. Also the ac unit has an on off switch anyways. Thanks for responding.

 

[IMM_Doctor]

Why not turn the unit off with the controls first?

BRILLIANT IDEA!

Again, Dr. Fine bings his wealth of common sense to the masses. Thank You!

Yes, even though we know the disconnecting means is rated to open while under load, why do it?

Our standard operating procedure is to stop the machine(s) and loads via controls first, then open the disconnecting means.

 

[ohmhead]

Well it doesnt matter but i normaly shut down the bucket or starter or vfd controls first .

If you dont have a starter then its a pretty small HP motor just turn off the switch or the breaker its not going to stress or blow up or flash over or have a problem in your life time ! I cant believe what iam reading in these post ?

The safety switch is made to turn it off hundreds of times read the specs from the factory or call SQ D thell tell you what i just said .
Now a breaker is not design to turn off or on hundreds of times you can damage it under load so turn off the starter then the breaker .
Or just flip the switch fused or non fused .
Oh dont forget to lock out & tag out while working on motor
Best to ya

 

[Karl H]

I always shut-down a machine using the factory controls before opening
a disconnect or breaker supplying the machine. The only time I do
"Hard-Starts" is to "Bump" a motor to check rotation.

 

[quogueelectric]

What's the difference?

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.

 

[wireguru]

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??

(no offense to our older, wiser members), how old was this guy? This seems like an episode of "IVE BEEN DOING IT THIS WAY FOR 35 YEARS!" Is he talking about the old switches where the handle directly operates a set of knife blades, and doesnt have the spring mechanism for instant open/close that modern switches have?

 

[quogueelectric]

(no offense to our older, wiser members), how old was this guy? This seems like an episode of "IVE BEEN DOING IT THIS WAY FOR 35 YEARS!" Is he talking about the old switches where the handle directly operates a set of knife blades, and doesnt have the spring mechanism for instant open/close that modern switches have?

If you whipersnappers would clean your ears out we wouldnt have this problem of arcflash!..

 

[iaov]

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. Each time such a device encounters an arc, it is diminished to some degree. Repeated operations under load will eventually result in the need to replace the device. OK, so you will tell me that they have been tested to 10,000 or more operations under load. My response is, why should we spend any amount of a component's remaining useful life, if we can avoid doing so by turning off the load first? We cannot know, when we reach for the breaker or the disconnect, what its internal condition might be. If we have the option of turning the load off or leaving it on, I submit that it is a better (and safer) habit to turn it off.

As to the original question about which is better to open first, the breaker or the local disconnect, my answer is whichever one has, through its operational lifetime, suffered the least amount of deterioration. :wink:

Well put Charlie. 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 scarey.