480V Molded Case Breaker Position Verfication

[joepower]

Molded case breakers in motor control centers are often operated indirectly by an external handle on the front of the MCC cubicle. As the mechanical linkages wear over time and the original circuit breakers are replaced with breakers that are not dimensionally identical, how do you ensure personnel safety by verifying that an external handle flipped to "off" has actually toggled the breaker to "off" and equipment is safe to work on? We presently have a technician open the door and check the load side of the breaker with a meter to ensure that it is de-energized, however, this is not a good place to be putting a meter since most breakers in combination starters are not rated to interrupt a fault at the load side of the breaker – you need to be on the load side of the contactor to match the way the device is tested for its certification. Also, one of the other threads at this web site indicated that they had molded case circuit breaker failures where the actual circuit breaker handle was toggled to "off", but the internal breaker mechanism did not move and the load side was still "hot" - is this a common occurrence? Thanks

 

[DanDel]

Everyone should always check for voltage with a meter on a circuit before working on it. This should be a standard safety routine for all electrical work.
That being said, I'm not clear on your explanation about why checking with a meter is not safe. If the C/Bs you are talking about are for the individual starter buckets, then these are tyically MCP(Motor Circuit Protectors), which look just like molded case C/Bs, but have only an Instantaneous function for a short ciruit trip. This will open for short circuit protection, not the contactor, which will only open for overload protection.
MC C/Bs can definitely fail(as can any device) and have the handle in the open position when one or more phases are still energized. This is why you should always check first with a meter(or two) before starting work.

 

[dpc]

If the operating mechanism can't open the breaker, you're going to have to open the door anyway.

My experience has been that it is usually pretty obvious if the breaker has opened, due to the snap action of the breaker. These breakers are specifically designed to have only two stable positions - full open or full closed, for obvious reasons. But it is also possible for one pole to fail to open or close even when the breaker mechanism throws over.

For motor starters with integral control power transformers, the OFF light should be out if the breaker is open. If the OFF light is lit, the breaker is closed.

But I agree with DanDel, you MUST verify the circuit is de-energized prior to doing any work. Fluke and others make an insulated non-contact voltage probe that can be a good FIRST step in checking for voltage. If the probe shows a dead circuit, you can verify with a meter.

And just to open another can of worms, the new NFPA 70E maintenance rules could require a fairly high level of PPE just to open the bucket door and do a voltage measurment.

 

[peebee]

PPE?

 

[dpc]

Sorry, my bad. PPE = Personal Protective Equipment

There are several levels defined, depending on the available fault energy where the work is being done.

At lower levels this is simply use of cotton clothing, including long sleeve shirts, along with gloves, safety glasses, etc.

At higher levels of fault energy, it ramps up to a full blast suit with face shield.

I think you may still be able to download the pre-print of NFPA 70E at the NFPA website. It has more detail than you could possibly want, especially on a Friday!

There is also a new IEEE standard (IEEE 1584, I think) that covers calculation of arc-flash hazards.

 

[joepower]

Regarding my statement that it may not be safe to be checking voltage at the load side of the breaker, the issue is similar to a typical 15A molded case breaker which is not rated/tested to interrupt at its load side terminal, there is always a minimum amount of wire in the UL test circuit . See

http://www.cadickcorp.com/tech002.htm).

Depending on manufacturer and vintage, the test circuit (particularly for series rated devices) may include the combined impedance of the breaker and the contactor (with the minimum resistance thermal overload in place). This was a warning that I picked up at breaker manufacturer’s seminar years ago and confirmed by looking up the UL test procedures.

 

[dpc]

You're correct that the UL testing for motor starters is based on a short circuit outside the starter. I believe that three feet of wire is used.

So for situations where the fault current is in between the "Unit" rating of the starter and the rating of the breaker alone, there is a risk of a failure of the breaker when attempting to interrupt the fault.

To classify the risk of taking a voltage reading as either "safe" or "unsafe" is requires a lot of subjective judgement. The only way to be completely safe would be to de-energize the entire MCC and place shorts and grounds on it.

Even if the breaker is within its rating, if an electrician creating a short circuit while taking a voltage reading would be subject to severe burns due to arc-flash radiant heat.

As I mentioned, there are insulated, non-contact voltage sensors that present very little risk.

 

[busbar]

joepower,

I had something very unusual happen to me about 15 years ago that resulted in a near fatal accident. As it turned out, I only suffered first and second degree burns on my hands. Before starting work, I firmly turned off a 90 amp three-phase breaker on a 480 volt oil well pump. After approximately 2 minutes, the breaker turned itself back on! As if that was not bad enough, I later found out that there were numerous breakers like this in the oil field and the oil company knew about them! My widow could have been rich, very rich!

This was first discovered by an oil well worker who heard a click when the breaker reset and went back to investigate. He was very lucky as this was an isolated well that enabled him to hear the click. Otherwise, he was about to open and do maintenace on the pump panel. The pump had a time delay on restart which caused the motor not to restart immediately.

 

[twosockets]

Busbar,

Somehow I have picked up your handle! I don't know how and I don't know how to fix it. I have e-mailed the host in hopes that they will fix it.

In the mean time, the above posting by busbar should instead be by Twosockets! Sorry about that!

 

[Toned]

Just a point of interest, but here in Canada, all molded case circuit breakers in use in MINES must have a clear window so a person can see the visible break of the contact tips. it also has to have a mechanism to lock the breaker in the open position