Mystery-why main C.B trips

[Toros]

Hi
I have a commercial building with 2000A serrvice
Any fault , shortage, happens in any tenant
the main 2000A circuit breaker trips instead of cir. brkers in the tenant sub panels


Do you know why??

 

[templdl]

Hi
I have a commercial building with 2000A serrvice
Any fault , shortage, happens in any tenant
the main 2000A circuit breaker trips instead of cir. brkers in the tenant sub panels


Do you know why??

Can we at lease know what the system voltage is, the make and model of the circuit breakers with the feature and well as the setting where they are set at so we at lease have a place to start? Also it would be extremely helpful if you know what time of day it trips is there is some sort of consistency. Are there any trip indicators on the breaker? If the breaker has ground fault that would be a good place to start.
Otherwise it is a crap shoot.

 

[charlie b]

There is no mystery here. It is a perfectly normal situation. Furthermore, there is nothing in the NEC that would require any preventive or corrective measures. And no, I am not pulling your leg.

Here is the basic truth: When a fault takes place in the tenant?s space, a large amount of current will flow. That current has to pass through the main breaker, the tenant?s branch circuit breaker, and any feeder breakers that lie in between the two. All of those breakers will see the fault current, and every one of them is going to react to it. All of them will be given the command to trip by their internal electronic or electromechanical protective elements. Whichever one actually manages to get open first will terminate the event, by stopping the flow of fault current, so that the rest of the breakers in the line will remain closed. Having said all that, what would make you think that any specific breaker (i.e., the one in the tenant?s space) will win the race?

The way to resolve this situation is to adjust the trip setpoints of the main breaker and any feeder breakers that are adjustable. You want the breakers further upstream to wait a bit before tripping, so as to allow the breaker closest to the fault to trip first. This concept is called ?selective coordination.? We are required by the NEC to include selective coordination for breakers in emergency (NEC 700.27) or legally required standby (NEC 701.18) systems. We are not required to do the same thing for the normal power distribution systems in dwelling units. The building owner might prefer that this be done, but the NEC does not require it.

 

[GoldDigger]

There is no mystery here. It is a perfectly normal situation. Furthermore, there is nothing in the NEC that would require any preventive or corrective measures. And no, I am not pulling your leg.

Here is the basic truth: When a fault takes place in the tenant?s space, a large amount of current will flow. That current has to pass through the main breaker, the tenant?s branch circuit breaker, and any feeder breakers that lie in between the two. All of those breakers will see the fault current, and every one of them is going to react to it. All of them will be given the command to trip by their internal electronic or electromechanical protective elements. Whichever one actually manages to get open first will terminate the event, by stopping the flow of fault current, so that the rest of the breakers in the line will remain closed. Having said all that, what would make you think that any specific breaker (i.e., the one in the tenant?s space) will win the race?

The way to resolve this situation is to adjust the trip setpoints of the main breaker and any feeder breakers that are adjustable. You want the breakers further upstream to wait a bit before tripping, so as to allow the breaker closest to the fault to trip first. This concept is called ?selective coordination.? We are required by the NEC to include selective coordination for breakers in emergency (NEC 700.27) or legally required standby (NEC 701.18) systems. We are not required to do the same thing for the normal power distribution systems in dwelling units. The building owner might prefer that this be done, but the NEC does not require it.

Often larger breakers, like your 2000A main, have field settable options for current, time delay, etc. As they come from the factory, these (except for current limit) will typically be set to their most conservative (sensitive, fast) values. If nobody did the engineering to determine what the correct settings were when the breaker was commissioned, it may well end up tripping before the downstream breakers on anything but a prolonged small overload.
Changing those settings is not a job for someone who does not understand all of the implications of those changes.
For a commercial building, with major inconveniences to tenants when the main trips, it is worth having the system checked out and tuned by a specialist. This may be something that a breaker maintenance contractor could do, or there may be a need to bring in a PE first. It is not my area of expertise.

 

[wyreman]

specialists come in and take quite a long time to work through the engineers specs and test waveforms and the like.

ground faults can take out large mains
that is normal and to be expected

a little spark in a streetlight can take out a large swath and that is how it is supposed to be

there is a ground fault dial or three on most every cb after a certain era
leave them be and call for implementation of co-ordination study

 

[GoldDigger]

specialists come in and take quite a long time to work through the engineers specs and test waveforms and the like.

ground faults can take out large mains
that is normal and to be expected

a little spark in a streetlight can take out a large swath and that is how it is supposed to be

there is a ground fault dial or three on most every cb after a certain era
leave them be and call for implementation of co-ordination study

I would not expect the main breakers in the tenant sub-panels to go out first reliably without engineered coordination, but if the branch breakers do not trip either it does get frustrating.
If the subpanel main and the branch circuits do not include ground fault protection, then that does leave only the main for those cases where the line to ground fault current is not enough to trip a pure OCPD very fast.
Installing GF protection at the tenant level may be the simplest fix in that case.

 

[xformer]

There is no mystery here. It is a perfectly normal situation. Furthermore, there is nothing in the NEC that would require any preventive or corrective measures. And no, I am not pulling your leg.

Here is the basic truth: When a fault takes place in the tenant?s space, a large amount of current will flow. That current has to pass through the main breaker, the tenant?s branch circuit breaker, and any feeder breakers that lie in between the two. All of those breakers will see the fault current, and every one of them is going to react to it. All of them will be given the command to trip by their internal electronic or electromechanical protective elements. Whichever one actually manages to get open first will terminate the event, by stopping the flow of fault current, so that the rest of the breakers in the line will remain closed. Having said all that, what would make you think that any specific breaker (i.e., the one in the tenant?s space) will win the race?

The way to resolve this situation is to adjust the trip setpoints of the main breaker and any feeder breakers that are adjustable. You want the breakers further upstream to wait a bit before tripping, so as to allow the breaker closest to the fault to trip first. This concept is called ?selective coordination.? We are required by the NEC to include selective coordination for breakers in emergency (NEC 700.27) or legally required standby (NEC 701.18) systems. We are not required to do the same thing for the normal power distribution systems in dwelling units. The building owner might prefer that this be done, but the NEC does not require it.

Couldnt the tenants CB be replaces with Fuses? Shouldnt that solve the issue?

 

[wyreman]

i prefer fuses!
but i dont install them that much...
in streetlights perhaps

:thumbsup:

 

[jim dungar]

Couldnt the tenants CB be replaces with Fuses? Shouldnt that solve the issue?

Probably not. Fuses are not always the best solution. especially when low level fault currents are involved.

The issue is coordination.
The OP needs to compare the Time Current Curves of his main device, including possible GF, with that of the Tenant's devices. Any overlaps may be able to be mitigated by setting changes or, as you suggested, device replacement.

 

[ramIII]

A call to the factory gear rep in your area may give you some help. In the absense of a coordination study they will usually give some parameters in the breaker adjustments not to go above which may help the situation.

I overheard a conversation in a electrical supply house the other day from a contractor who was having the same problem. Another electrical contractor told him he just sets everthing to the highest settings and the problem goes away. The first contractor says that's what he will do to see if it helps. I told him to make a call to the factory rep in our area to see if this was wise.

 

[cslater]

Yeah a fuse probably will act slower but they are also a pain when you have to replace them - especially if you don't have any on hand!

Coordination studies don't have to be that expensive - especially for a relatively small system. It's worth having a PE look at though, because there are other implications to the settings, such as Arc Flash risk and potential damage to equipment.

 

[templdl]

Often larger breakers, like your 2000A main, have field settable options for current, time delay, etc. As they come from the factory, these (except for current limit) will typically be set to their most conservative (sensitive, fast) values. If nobody did the engineering to determine what the correct settings were when the breaker was commissioned, it may well end up tripping before the downstream breakers on anything but a prolonged small overload.
Changing those settings is not a job for someone who does not understand all of the implications of those changes.
For a commercial building, with major inconveniences to tenants when the main trips, it is worth having the system checked out and tuned by a specialist. This may be something that a breaker maintenance contractor could do, or there may be a need to bring in a PE first. It is not my area of expertise.

Your preaching to the choir here.
Can we at lease know what the system voltage is, the make and model of the circuit breakers with the feature and well as the setting where they are set at so we at lease have a place to start? Also it would be extremely helpful if you know what time of day it trips is there is some sort of consistency. Are there any trip indicators on the breaker? If the breaker has ground fault that would be a good place to start.
Otherwise it is a crap shoot.
But it is fun show how much we know and to theorize and to guess as to why the main trips when there is so very little information provided. If I were still in a possition of an application engineer there is no way that I could assist this customer unless he would work with by by providing answers to my questions about the breaker after which I could narrow down the possible causes of trips. I have not seen anything that would allow me to assist the OP in determining what event tripped the breaker. Then evaluation of the load for a cause of trip.

 

[charlie b]

Another electrical contractor told him he just sets everything to the highest settings and the problem goes away. The first contractor says that's what he will do to see if it helps. I told him to make a call to the factory rep in our area to see if this was wise.

It is not wise. Here is why:

It's worth having a PE look at though, because there are other implications to the settings, such as Arc Flash risk and potential damage to equipment.

It a breaker upstream takes longer to open, because you set it at the highest delay setting, then any arc flash event that takes place will continue longer, before the upstream breaker finally terminates the event by opening. That will mean a greater amount of arc flash energy will be released. So blindly changing settings is not wise. Some thought needs to go into the setpoint decisions.

 

[ramIII]

It is not wise. Here is why:

It a breaker upstream takes longer to open, because you set it at the highest delay setting, then any arc flash event that takes place will continue longer, before the upstream breaker finally terminates the event by opening. That will mean a greater amount of arc flash energy will be released. So blindly changing settings is not wise. Some thought needs to go into the setpoint decisions.

I knew it was not wise. If he didn't have any idea of the ramifications of blindly changing settings I did not want to be the one he came back to when his switchgear blew up or somebody got hurt saying well you told me to do this. The gear reps in our area will usually help with the settings. They will need some info to be able to accomplish this.

 

[templdl]

It is not wise. Here is why:

It a breaker upstream takes longer to open, because you set it at the highest delay setting, then any arc flash event that takes place will continue longer, before the upstream breaker finally terminates the event by opening. That will mean a greater amount of arc flash energy will be released. So blindly changing settings is not wise. Some thought needs to go into the setpoint decisions.

This is what separates the men from the boys, that is knowing that you don't mess around with setting and recognize that the entire system most be considered.
I use a very simple illustration of an electrition the used #12 wire because of derating reasons for a 15a ckt protected with a 15at breaker. The next guy comes alone seeing #12 on a 15at breaker and conclude that the ckt is good for 20a and changes the breaker to 20at.
Bottom line is that the electrician must be able to actually think and use reasoning.
Instantaneous settings on a breaker usually are not any big deal as that is very difficult if impossible to coordinate anyway. But, when there is a pick and delay setting or short time pick up and/PR delay those settings commonly are there for coordination with other devices an are not to be messed with unless the rest of the system is considered by someone who is versed at system coordination.