Vacuum Contactors as Isolators

[jraef]

In a project I am working on for 15KV equipment, one of the user's engineers has required us to supply vacuum contactors to isolate a section of equipment for service purposes. These would NOT be draw-out cubicles, this is Metal-Clad Load Interrupter Switchgear. I have always operated under the premise that vacuum contactors are not inherently safe as service isolation devices, and that was because they can "leak". I have been told by others that this is also backed up by OSHA and/or ANSI, but the only thing I can find close to that is under the basic OSHA LOTO clause, because you can't Lock-Out/Tag-Out a fixed vacuum contactor. I have also checked the literature of several Vacuum Contactor mfgrs and find no reference to leakage.

Any thoughts? I am looking for documentation to convince this engineer that this is a bad idea.

"Venditori de oleum-vipera non vigere excordis populi"

 

[RalphChristie]

jraef:

Do they just want to use a contactor to isolate equipment to work on?

Sorry, I have no references, but in my opinion this is NOT a good idea. I agree with you, I would certainly not recommend it. On all our MV equipment we have either a contactor with additional knife-switches, or Circuit-Breakers with additional knife-switches. Maybe a little more expensive, but you can't measure someone's life with money. And if I am the guy who's going to work on it, I want to see the power is off and locked out.

Just my opinion...

Regards
Ralph

 

[Lakey]

Jraef:

I can only speak for LV applications, but I agree with your thinking.

I have inspected vacuum-sealed relays/contactors after switching selected loads and have witnessed significantly reduced dielectric strengths between the main terminals. The burnt contact material tends to coat the internal walls thus providing an electrical path. I’ve seen breakdowns as low as 350vac!

Best regards,

 

[RalphChristie]

The question you have to ask is if they (Engineers) are willing to work on that equipment if the power is off and there is just a contactor to isolate the power.

 

[jraef]

"Do they just want to use a contactor to isolate equipment to work on?"

Yes! That is precisely the problem I have with this. I know it is a bad idea, everyone I work with knows it too, but the end user has apparently seen a proposal from someone else that shows it. So he thinks it is OK, and I am in the unfortunate position of having to impugn the validity of someone elses poor design idea.

"Venditori de oleum-vipera non vigere excordis populi"

 

[buzzp]

Ouch, surely there must be something in the companies lockout tagout procedures against this? If the procedures exist. I know the standard agencies don't tend to 'dumb' stuff down to address this situation but maybe they did.
Not good and if I were the electrician with my hands near this I would refuse to touch it.

 

[Cotton]

I agree with everyone's thoughts. However, the lockout tagout procedure should require installation of grounding clusters on anything above 5 kV if hands on work is required. Even with a grounding cluster, I would not be comfortable working on a 15KV system and not being able to visually verify a switch is open.

Just a note on vacuum contactors: We have several in our plant and they do fail frequently.

 

[aolalde]

I have seen some motor control centers were the disconnecting means is a contactor on a cart. For service of the motor or lines downstream, the contactor is disengaged and removed out of the panel, and then the panel doors are safe locked by the electrician, he keeps the key to guarantee no accidental circuit re-closing. A warning label must be attached giving instructions for disconnecting and locking.

 

[ScottyUK]

In the UK, intrusive work on the electrical parts of HV equipment requires the isolator(s) to be locked open and the circuit earth(s), or busbar earth where the work is on the switchgear, to be locked in the closed position. In some instances it may be necessary to apply local drain earths over and above the circuit earth.

For work which is limited to mechanical equipment associated with the HV equipment - maintaining a pump driven by the HV motor for example - earthing is not usually required.

Quite how use of a fixed vacuum contactor achieves either of the isolation or earthing requirements beats me. Your client must have limited knowledge of safe working on HV equipment.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[ruggedscot]

Quite simply vaccum contactor is not a recognised form of isolation. No way.

You need to have proper isolation and the HV circuit properly earthed down before working on any parts of
the normally energised apparatus.

Rugged

 

[QUAVIET]

Again I am educated at this forum. I did not know what a vacuum contactor was before. Does not the NEC cover this about a physically lockable disconnect on the motor branch circuit? The user's engineer you mentioned must be right out of an electrical correspondence school and had someone else do his homework.

 

[jraef]

Thanks everyone for the feedback.
I was hoping to connect with someone who knew of a document I could point to other than the OSHA rule, but have not been successful myself. So at least I am at ease as to the fact that this is the best approach. We are "hanging our hat" on this as the prima facie issue, also noting some other technical problems it poses for our components.

I would not catagorize the consultant as anything less than prfessional, just notup on all the ramifications of sucumbing to someone else's whims. As I understand it this request came from the utility's engineering department and is based upon the fact that several of the Medium Voltage VFD manufacturers seem to provide this scheme as a means to service their inverter sections. Although we are not providing a VFD, the concept apparently carried over in their minds.

Ahhh... the dangers of establishing a precedent!

"Venditori de oleum-vipera non vigere excordis populi"

 

[tinfoil]

Rule 36-214 of the Canadian Electrical Code (as well as the Ontario Electrical Safety code) states that there must be a means of visible inspection of the breaker contacts, or a draw-out mechanism.

On top of this, provincial practices require the application of grounds before work may be done.

 

[tinfoil]

Have a look at

http://www.us.squared.com/us/produc...AFA85256A3A005759E7/$file/visivacFrameset.htm