I went to a site to adjust breaker settings for a client per our coordination study and had a breaker trip after increasing the long time delay settings. The breaker had load on it and is a Square D type NX breaker. After reclosing the breaker, it tripped again while increasing the short time delay setting. The breaker did not trip while adjusting the pickup settings. I can understand if it tripped while decreasing the settings but I find it odd that it tripped when increasing the settings. At first I thought it was just the single breaker that had an issue but we tested it on 2 other Square D type NX breakers that were feeding chillers and it did the same thing. The site is not a critical system so we plan on going back during the weekend to make the rest of the setting adjustments. I'm assuming the breaker has a sensor that trips the breaker when the long time and short time delays are adjusted. Has anyone had experience with changing settings on these type of breakers? The client would like to know why it tripped and I have no explanation for him. Any help would be appreciated.

There is at least one manufacturer of protection relays that has this "feature" built into their relays. I do not remember which one.

All the breaker manufacturers have lawyer clauses that tell you that adjusting settings can cause a breaker to trip. Not all of them do it in practice. This puts you in a catch-22 situation because you never know when a unit might be designed to intentionally trip during normal operation until it has been tested.

As I recall the manufacturer claimed that this was a "feature" where in the event that something happened to the trip circuit or that someone "accidentally" changed a setting, it would trip. This made their circuit breakers "safer". With discrete protection relays in particular I have seen cases where a bad connection developed with an electro-mechanical relay at the pin. I believe that it turns out that when this occurs the particular relay in question reverted to the highest setting. One can easily see in this case how it would be a "feature" if it reverted to the lowest setting, even if it meant causing a nuisance trip if someone tried to move the pin. At least with old electro-mechanical protection relays though they are so notoriously unreliable that the switchgear nearly always has switches that allow the relay to be removed and tested without tripping the circuit breaker itself.

Your answer is you didn't expect this to happen. Did you? If you did, why would you make these adjustments with the power on? In my experience this is fairly rare.

The point is most manufacturers will tell you to shut off the breaker before adjusting the settings. I think with solid state trip breakers will go to the minimum settings as you move the selector from one setting to another. If the device did go to its minimum and you have a load that exceeds the minimum setting, you're likely to get a trip. However, it might be wise, if these are critical breakers, to have the breakers checked. I believe the trip units are equipped with a port for a test kit to simulate trip currents. If you go that route, you might find there an issue.

I'd reach out to Jim Phillips as he worked for Square D. I think has some experience with this.

As a further follow up, think about the result as well. With an electronic trip unit changing a value changes an internal register that is used to compare to the reading...basically we're changing a number. In the case of solid state and electro-mechanical relays we are changing a mechanical mechanism, even if it's just a pin. I have not seen anyone testing relays at every setting of the relay...they are normally tested only for the "as found" settings.

So with the microprocessor types, adjusting the number in the settings does not affect operation of the device so no surprises there. However with the other types there is a small chance that changing the trip setting could in the case of a pin stop making good contact which in turn can cause it to trip earlier or later than it should. So adjusting the device should be followed up with verifying that it still trips properly after the change. The rate that the new setting doesn't work is pretty high from my experience.

I had this happen with another manufacturer's solid state trip breaker in the 1990's. wilhendrx is correct, that when the setting was moved from low to high, it tripped (unexpected and always a heart stopper) It was found that the setting dropped to zero when going from one setting to the next. It was the first generation of these breaker's trip units (not mentioning names).