Can a high amperage molded case circuit breaker instantaneous setting be changed HOT? 2

[bdn2004]

We found that an 800A 208V main breaker in a panel with an electronic trip unit should be set to a different instantaneous setting. The answer we've got is NO, this could cause a nuisance trip. Is this a true statement? Specifically a Square D Type PG with a Micrologic 3.0 trip unit.

 

[jraef]

It should not trip just from the act of changing it. There are now what are called "Maintenance Mode" breakers in which the instantaneous trips can change on command, such as when someone enters a room, in order to reduce the incident energy for arc flash mitigation.

The ACT of entering a panel when live to adjust a breaker setting is in and of itself a very risky endeavor now, so that may be of concern, but I don't think making the change will create a trip scenario. Square D would be the ones to answer that definitively though.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[rcw retired EE]

Depends. I have changed settings under load on various breakers without a spurious trip. But it was done with production knowing the risk of the breaker tripping & personnel on standby. If the rotary switch open circuits or creates noise during the switching it could fool the circuit and trip. With today's knowledge of arc flash, I would not do it hot.

As a side note, in the 1970’s, two workers were killed adjusting a MCCB instantaneous trip on a panelboard main breaker at a telecommunications facility. The adjusting screws for each phase were part of the magnetic trip assembly integral to the trip unit. The panel was bottom fed with the main on the bottom, reverse fed. Adjusting screws were still energized with the breaker was open. Safety relied on the insulating quality of the molded, slotted knob on the face of the breaker. A metal cover partially obstructed the adjustment knob’s slot.

To insert the driver into the slot, the electrician had to pry on the plate's edge. The insulating button cracked and a ground fault ensued through the screwdriver. I don't recall if it was a service entrance with slow or no upstream protection or a feed from a switchboard. The arc killed the electrician and his co-worker.

The company had many installations using the same design around the country. They hired local testing companies to inspect the similar panelboards for the same issue. We found and mitigated a couple instances in our area.

If you’re adjusting MCCB trips, do it dead and make sure it is not reverse fed!

 

[dpc]

While I've done this many times in the past with no nuisance trip, today I would be reluctant to do it. I'd inquire with Square D. I'm guessing they will recommend de-energizing. If you have to expose live parts to access the breaker, It would require an Energized Electrical Work Permit per NFPA 70E, and I don't see how you would justify doing it energized. Schedule a short outage and exercise the breaker while you're at it.

 

[bdn2004]

This will be done de-energized. I’m just checking to see if anyone can back up the claim that the reason it’s not done hot is because it could trip. This is an electronic trip unit. And the adjustment screws are on the front, always exposed.

It would seem in our safety conscious time there would be warnings galore on the breaker to not adjust while it’s energized. But I don’t see any.

 

[dpc]

I'm sure it's covered in the instructions that are never read by anyone.

 

[waross]

Ask your company safety advisor if it is safe. We are all pretty sure of the answer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[oldfieldguy]

Can be done, usually. I've explained this to clients before - "it PROBABLY won't trip, but... Do you want to risk it?"

However, things we used to do in the 'good old days' are safety no-nos today.

Company 'safety advisor'? You mean like the rocket scientist who told powerplant n=management to shut down and lock out a 60 MW steam turbine generator so brushes could be changed?

old field guy

 

[Zogzog]

Some electronic trip units will default to the minimum setting when you change the settings (Between settings), I believe the micrologic series will do that, so depending on load the breaker may trip. Best to do it de-energized.

 

[jghrist]

Using a maintenance mode switch to change instantaneous settings or a trip unit with a maintenance mode setting would require changing settings while hot. De-energizing to put an OCD into maintenance mode would seem to defeat the purpose. If you can de-energize the circuit, then you should do the maintenance de-energized.

 

[Zogzog]

Yeah but the maintenance mode selector switch is typically mounted externally on the door or a remote location.

 

[jghrist]

The location of the maintenance mode selector doesn't affect whether or not it causes a nuisance trip. I think this was the concern of the OP. If the breaker is well maintained, a nuisance trip would not constitute an arc risk.

 

[bdn2004]

I know it doesn’t seem like a big deal...but shutting down a main panel in a large plant that runs continuously does require a fair amount of planning and expense and people’s time. Documents gotta get signed and different managers have to be alerted etc. We have to attend the weekly meeting to get it on the schedule. And the work is usually done on the weekend premium time.

When all we are doing is turning the screw that’s located exposed right on the front of the panel just 1/4 inch ... seems like it’s at least worth a question and a definitive answer.

 

[dpc]

Did you ask the breaker manufacturer? That would be the definitive answer.

 

[bdn2004]

Since everyone on here, every field electrician I’ve asked, every person in authority at the Plant is already absolutely convinced it’s practically a life and death risk to touch that dial while HOT - I dropped it. Why fight?

It’s gonna take more than a phone call. I need documentation.