How to safely disconnect power supply to electric motors? 4

[M.Barzegar]

Hello everyone.
We have several 400V MCCs in our factory with withdrawable unit cubicles (SIVACON 8PT). The current practice for disconnecting power supply to electric motors in our factory is that we switch off the MCCB and we also withdraw the power unit completely. Because of the high number of connect/disconnect, these power units are starting to show some mechanical issues like in connectors and contacts.
Recently it was proposed to prevent these issues, we should only switch off MCCB but leave the power units in position. This decision was contested by our factory’s Health and Safety group because they believed it is not completely safe as technicians who perform maintenance on these electric motors are extremely exposed and an unwanted start of motors could cause even death.
My question is what is the standard approach in your facility? And is there any standard or reference I can refer to on this subject?
Thanks a lot.

 

[RRaghunath]

Part of the 'Permit to Work' procedure, it is standard to withdraw the module before allowing anyone to work on the cable / motor.
What surprises me is - how frequent do you work on motor? Why should the connectors getting damaged be a concern? Better to look in to those aspects?
Also, whether racking-in and out of the module is being done by trained technicians?

 

[LittleInch]

The issue would seem to be how can you secure the isolation of the motor in the MCCB. If you have no lockout facility then this would not be acceptable. What does your MCCB look like?

The vendor should have recommended practices in the maintenance manuals.

These sorts of issues are often in local safety regulations or guidance. Whichever country you are in you need to review what their requirements and procedures are.

There is no doubt that physical removal or disconnect of supply is lower risk than a simple lock out switch, but if operated correctly those safe systems of work can be acceptable.

How can the motor be started if the supply is disconnected at the MCCB?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[M.Barzegar]

@RRaghunath
Thanks for the response. This happens to some of our motors and their modules that are designed poorly and require more maintenance. Also some of the conveyors need regular maintenance and for that we need to isolate power supply to the motor frequently.
Rack-in & rack-out is done by trained technicians but the frequency is a bit too much and leads to these issues.


@LittleInch
Appreciate the response. We switch off the MCCB via the rotary handle on the surface of module which is coupled to MCCB and we put a lock on it (something like the picture attached below).
The two main concerns are if MCCB malfunctions and doesn't disconnect the supply and technician doesn't notice or worse if after disconnection, MCCB malfunctions and somehow allow the electricity to flow the maintenance technicians are in great life-threatening danger.

 

[rand1234]

Interesting. At our facilities, depending on the time of installation, the motor may or may not have a local disconnect for isolation purposes. If there is a disconnect, than of course, that disconnect has to be opened as part of the clearance procedure before any work can begin. But if there is no disconnect, the circuit breaker in the MCC bucket is opened and a 'do not operate' (DNO) tag is applied. In addition, the cables are lifted on the bucket side and either taped or booted for added protection. And the main thing is we don't have any withdrawable unit cubicles for our MCC's.

 

[BrianPetersen]

Stop it (Press the equipment "stop" button)
Lock it (switch off the designated disconnect switch and apply a padlock)
Test it (attempt to turn the equipment on, or use test instruments to verify that the power is off, as the case may be)

If there is a local disconnect switch, in which the cable between the disconnect switch and the motor in question is clearly visible and can be inspected, that is the best.

The span of control of the disconnect switch should be adequately communicated in the lock-out procedure and with suitable labels or signs on the machine.

I've never seen anyone actually remove the power unit from a MCC panel as part of a lock-out procedure. It seems to me that this operation itself adds its own element of risk. If someone has mixed up cables between the MCC panel and the mnotor, this isn't going to help.

 

[LittleInch]

Your concerns appear quite bizarre.

If the MCCB doesn't disconnect the supply then it is faulty and needs to be replaced. As Brain says, the first thing you do after locking it off is to test the contacts by taking the JB plate off or checking that it doesn't start.

How exactly an MCCB which is locked off as you show can then reconnect the supply is not a credible event. If you're trying to eliminate things like that then I think you've gone too far into the realms of what if...

I have seen MCCs removed and left disconnected and it is a very visual sign that something is being worked on, but equally there is nothing then to stop someone putting it back in place before they should. It's difficult to avoid a padlock, especially if the person working on the motor has the key...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[crshears]

My preferred option: Check MCCB open; open, lock [using a hardened shackle lock, not just a brass one] AND TAG DNO [ Do Not Operate] local disconnect at motor; put the key in your pocket. Highly unlikely any one will have the opportunity to mess with the isolation without your being aware of it.

No local motor disconnect? Check MCCB open; rack it out to the disconnected position; lock [with lock as above] AND TAG DNO. Disadvantage is you're not constantly monitoring your isolation point, so the trust factor enters into it [ is the organization a "cowboy" one - no offense to cattle herders intended - or are there "cowboys" working there who will cavalierly switch isolated equipment into service?

My utility has a policy that following even brief absences from the work location the isolation is to be re-checked for integrity [ although how this check is to be performed is not specified].

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[che12345]

Dear Mr M.Barzegar

M. "... factory with withdrawable unit .... The current practice for disconnecting power supply to electric motors in our factory is that we switch off the MCCB and we also withdraw the power unit completely. Because of the high number of connect/disconnect, these power units are starting to show some mechanical issues like in connectors and contacts."
C1. My, a "compromised" opinion:
a) where there is a [local isolating switch]: Switch off the MCCB, lock+tag the MCCB handle in the off position. Switch off the [local isolation switch]. Open the cover to check ensure that both the [incoming and outgoing] terminals are dead. Cover the switch and lock+tag the handle in the off position. Attn: The door shall NOT be able to open when the handle is locked in off position.
b) where there is no [local isolating switch]: Switch off the MCCB, lock+tag the MCCB handle in the off position. Withdraw the withdrawable unit to the withdrawn position. Lock+tag the mechanism to prevent it from moving to the [service] position.
C3. Atten: In any case, DO NOT [leave the cell open] (i.e. without the tray in it). This is to prevent entry of any rodent/snake etc. Always cover it up with a spare unit or with a blanking cover; if the unit is required to be removed for service etc.
Che Kuan Yau (Singapore)

 

[Padlock01]

rand1234
As per your illustration you have correctly lockedout the power to the motor at the source (MCC).

 

[che12345]

Dear Mr rand1234

I am of the opinion that your advice is " ideal" but NOT practical to disconnect the wires etc., consider that it is a withdrawable system. It could be that you don't have any withdrawable unit cubicles for your MCC's.
Che Kuan Yau (Singapore)