Electrician Talk banner
1 - 5 of 5 Posts

·
Swimmer
Joined
·
682 Posts
Discussion Starter · #1 ·
I worked on a 3-phase knife switch style disconnect with fuses.
2 of the fuses were blown. The remaining phase had what I think was a carbon deposit on the blade and receptacle into which the blade seats when the knife switch is in the on position. There were no signs of heat damage on plastic or wires. This dirty blade was also pitted. I sand papered the contact, put in new fuses and everything works.

I ohmed each phase to ground and to each other. Results: complete open to ground and 1 ohm to each other.

I measured 10A on each phase while the motor was running. The tag says 24A and the vacuum, that the motor operates, worked as advertised.

There were also control circuit connection problems that I assume had nothing to do with this deposit.

Questions:
1. Is this a typical disconnect failure mode?
2. Could this deposit have been caused by some problem in the motor that will return?
 

·
RIP 1959-2015
Joined
·
39,618 Posts
I worked on a 3-phase knife switch style disconnect with fuses.
2 of the fuses were blown. The remaining phase had what I think was a carbon deposit on the blade and receptacle into which the blade seats when the knife switch is in the on position. There were no signs of heat damage on plastic or wires. This dirty blade was also pitted. I sand papered the contact, put in new fuses and everything works.

I ohmed each phase to ground and to each other. Results: complete open to ground and 1 ohm to each other.

I measured 10A on each phase while the motor was running. The tag says 24A and the vacuum, that the motor operates, worked as advertised.

There were also control circuit connection problems that I assume had nothing to do with this deposit.

Questions:
1. Is this a typical disconnect failure mode?
2. Could this deposit have been caused by some problem in the motor that will return?
Almost every time I see that it is usually water or condensation that is the cause.
When a blade like that becomes corroded it eats away at the metal causing a loose connection and then small arching,That arching eats away more metal so that is how it became pitted.

However that may not be why the fuses blew,If it was just one fuse on that blade then that would probably be the cause .

The problem may return if that blade still has a loose connection you should check on it and see if it is heating up more than the other blades.
 

·
Swimmer
Joined
·
682 Posts
Discussion Starter · #3 ·
However that may not be why the fuses blew,If it was just one fuse on that blade then that would probably be the cause .

Since the phase with the deposit completely lost continuity, my theory was that the remaining phases took on the additional load causing them to blow... but I'm not a 3-phase motor expert.
 

·
Registered
Joined
·
31,944 Posts
I worked on a 3-phase knife switch style disconnect with fuses.
2 of the fuses were blown. The remaining phase had what I think was a carbon deposit on the blade and receptacle into which the blade seats when the knife switch is in the on position. There were no signs of heat damage on plastic or wires. This dirty blade was also pitted. I sand papered the contact, put in new fuses and everything works.
Movable Contacts and stationary contacts.
I ohmed each phase to ground and to each other. Results: complete open to ground and 1 ohm to each other.
With a megger, if not your test proves little to nothing other than a dead short.

Did you perform an FOP test?

Questions:
1. Is this a typical disconnect failure mode?
2. Could this deposit have been caused by some problem in the motor that will return?

Blowing two fuses is usally indicitive of a fault phase to phase.
Carbon is a conductor and can result in phase to phase and phase to ground faults.

With all power off I would have meggered the FSS

Loads side and line side, phase to phase and phase to ground, then line to load with switch open.
 

·
Senior Member
Joined
·
13,020 Posts
If one pole in the disconnect was f**ked up and not making good connection, that would cause the arcing and pitting damage. If it lost continuity altogether and single-phased your load, that would cause the other two fuses to blow (if it's a motor)

Sandpapering the pitted/damaged contacts isn't going to fix anything. You'll start seeing problems in a few weeks or months. Once that damage starts you need to replace the burned up components if possible. The contact points depend on being properly seated against each other, if you sandpapered a bunch of material off then the problem will recur.
 
1 - 5 of 5 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top