meter main breaker tripping instead of sub main breaker.... (3 phase)

[techniphase]

i have a meter socket main breaker200 amp 3 phase 120/240 in an industrial shop. it has been raining really heavy prior to the call , no problems till rain. ( COINSEDENCE?). overhead feed.

The meter main breaker is about 80 ft from the sub panel. the sub panel also has a 200 amp main breaker, why would the sub main not trip and the socket meter main trips when using a particular machine in the shop..keep in mind, there has been no alterations or additions to this panel for years, it just started last week.

havnt did any load readings yet , just changed out the two main 200 breakers and still getting same problem...

any suggestions well apreciate....

 

[petersonra]

It is likely that even if they are the exact same breaker that one of them would trip before the other. Just random.

I doubt the rain has anything to do with it. Might be that when it rains there are more things on line like sump pumps.

 

[techniphase]

thought maybe the transformer or poco problem , just odd to me when the main trips but nothing in the sub panel or its main trips....

 

[ceb58]

i have a meter socket main breaker200 amp 3 phase 120/240 in an industrial shop. it has been raining really heavy prior to the call , no problems till rain. ( COINSEDENCE?). overhead feed.

The meter main breaker is about 80 ft from the sub panel. the sub panel also has a 200 amp main breaker, why would the sub main not trip and the socket meter main trips when using a particular machine in the shop..keep in mind, there has been no alterations or additions to this panel for years, it just started last week.

havnt did any load readings yet , just changed out the two main 200 breakers and still getting same problem...

any suggestions well apreciate....

Could it be getting water into the conduit or sheath between the panels and it is finding a small bad spot in the wire?

 

[jim dungar]

thought maybe the transformer or poco problem , just odd to me when the main trips but nothing in the sub panel or its main trips....

It happens quite often.
It often seems very few 'part-time' designers of electrical systems consider coordination of protective devices except for 'emergency' systems.

 

[templdl]

It happens quite often.
It often seems very few 'part-time' designers of electrical systems consider coordination of protective devices except for 'emergency' systems.

Jim, I know all too well that I'm preaching to the choir with you as you know this stuff.
As such my response is directed to those that are not familiar with ways that they can atainobtainIf you have two devices in series and the instantaneous fault is scene by both a 15at branch breaker and a 100at main breaker it is pot luck which one will trip should the magnitude of the fault be greater than the trip point of either breaker. There is no coordination.
With commercial/industrial breakers that have electronic trips units with adjustable pickups and delays it is possible to adjust the upstream and down stream breakers such that the up stream breaker has enough delay that if gives the down streak breaker time enough to clear the fault first. Even with breakers that do have basic adjustable magnetic pickups there is no assurance of coordination.
With the common instantaneous breaker instantaneous is just that, instantaneous. Should there be an instantaneous spike of current that rises to be equal to or greater than the instantaneous pickup of any breaker in series with the fault anyone could trip although we wish that the breaker which is closest to the fault would trip first. Of course there are also breakers that have the ability to communicate with one another to assure that the down stream breaker sees the fault and allows a brief delay for the down stream breaker to trip. If it doesn?t then it trips.

And we all know a lot of this stuff is price driven. Even though a designer may have the where with all to coordinate a system the people paying for the installation may want to go the cheapest route.

 

[techniphase]

but why...why is it tripping when they havnt added any circuits or equipment. this has been happening a week ago and its been okay for years. only when they are running certain machines it does it.

im going this week to get a load calc for each phase at the main and each phase before the sub main and eliminate that possibility...ill post if i find an issue.

 

[brian john]

but why...why is it tripping when they havnt added any circuits or equipment. this has been happening a week ago and its been okay for years. only when they are running certain machines it does it.

im going this week to get a load calc for each phase at the main and each phase before the sub main and eliminate that possibility...ill post if i find an issue.

This is not magic there is a reason for the tripping, why one circuit breaker over the other has been explained, not isolate the cause of the tripping.

Megger the feeder,
Perform FOP across both circuit breakers
Take amp readings when the machine is starting
Ideally but expensive do long term current recordings.

 

[Strathead]

but why...why is it tripping when they havnt added any circuits or equipment. this has been happening a week ago and its been okay for years. only when they are running certain machines it does it.

im going this week to get a load calc for each phase at the main and each phase before the sub main and eliminate that possibility...ill post if i find an issue.

Techniphase's information is correct, but it goes deeper. First, a good rule of thumb is 10 times the breaker trip rating for "instantaneous trip". Per Mil spec, instantaneaous is 5 cycles or less. Armed with this information, you need to start troubleshooting. You are referring to the breaker tripping when utilizing certain pieces of equipment. 10x the rating of the 200A breaker is going to be FAR more amperage than the draw of your building, let alone the main's rating. You have either a direct ground fault or a direct phase to phase short. If your 800A is always the one that is tripping, I would expect the short to be on the line side of the 200A breaker, but that is not science, it goes back to the random nature of instant trip that Jim cited. At least on occasion the 200A should trip before the 800A. I would start with some megger readings though. Basically, I am indicating that overload, which you are implying in your initial comments isn't the cause of either breaker tripping.

 

[JoeStillman]

Do the identical breakers have adjustable instantaneous settings? Both breakers are seeing the same fault (or inrush) current. If the upstream breaker has a lower setpoint, it will be the first one to trip.

Most molded case breakers are shipped with the instantaneous trip current set to minimum. Don't be afraid to set it higher if you need to. If you set the upstream instantaneous higher than the downstream, you have a better chance of coordination.

More importantly, why is the machinery suddenly causing overcurrents? If utility voltage is running low, you can have higher motor currents. Maybe a tap change somewhere is in order. Check your power factor. Capacitors can help to boost motor voltage and reduce running currents.

 

[PEDRO ESCOVILLA]

only one machine in the shop is causing the trip? whats the machine? hows the motor winding ?

 

[Strathead]

Do the identical breakers have adjustable instantaneous settings? Both breakers are seeing the same fault (or inrush) current. If the upstream breaker has a lower setpoint, it will be the first one to trip.

Most molded case breakers are shipped with the instantaneous trip current set to minimum. Don't be afraid to set it higher if you need to. If you set the upstream instantaneous higher than the downstream, you have a better chance of coordination.

More importantly, why is the machinery suddenly causing overcurrents? If utility voltage is running low, you can have higher motor currents. Maybe a tap change somewhere is in order. Check your power factor. Capacitors can help to boost motor voltage and reduce running currents.

This is not really true. Once the current goes over the instantaneous set point, the breaker will trip as quickly as the internal mechanism allows it to. For all intents and purposes, it doesn't matter how far over the trip rating this is. For some reason, I had it in my head that the main breaker was larger than the sub panel breaker, but it still doens't matter. I am making the assumption that the piece, or peices of equipment are not causing an overload condition, first, the individual breaker, or the thermal overloads in teh equipment should trip and second, with a 200A overall service, the equipment is probably not in the 100A load range.

 

[jim dungar]

And we all know a lot of this stuff is price driven. Even though a designer may have the where with all to coordinate a system the people paying for the installation may want to go the cheapest route.

I do not believe that 'selective coordination' must be acheived in all installations. For the vast majority of installations, there is little reason to go through the costs and the hassels.

But, people choosing protective devices should put some thought behind their selection rather than just the Long Time rating. If they did, we would have fewer of these 'why did the upstream device open' questions.

 

[JoeStillman]

This is not really true. Once the current goes over the instantaneous set point, the breaker will trip as quickly as the internal mechanism allows it to...

I was assuming that the change in current was not instantaneous. Even though we talk about "instantaneous" set points on breakers, faults don't go from 0-10kA instantaneously. It takes a half cycle or so to max out. There's a chance the downstream breaker could win that race.

We're assuming the breakers are tripping on instantaneous. They could be going out on thermal. If one breaker is in a hot environment and its downstream twin is not, the hot one will always trip on overload before the cool one.

 

[templdl]

Do the identical breakers have adjustable instantaneous settings? Both breakers are seeing the same fault (or inrush) current. If the upstream breaker has a lower setpoint, it will be the first one to trip.

QUOTE]

This is not necessarily so. If the magnitude of the fault is high enough it would be pot luck which breaker will trip. Sorting out what happens when there has been a fault is basically all but impossible. This ofter happens when there is a fault on a 15at branch breaker an a 1ooat main trips.
Yes, assuring that a down steam breakers instantaneous is lower or has been set lower if adjustable that an upstream device is far from being a 100% solution for coordination.

 

[templdl]

I do not believe that 'selective coordination' must be acheived in all installations. For the vast majority of installations, there is little reason to go through the costs and the hassels.

But, people choosing protective devices should put some thought behind their selection rather than just the Long Time rating. If they did, we would have fewer of these 'why did the upstream device open' questions.

I agree entirely.
It depends how deep the pockets are or the necessity of the importance of a continuity of service when balanced against the odds that an even should occur.
Until an understanding and an appreciation is developed for how an OCPD responds to arcing and bolted faults there will be those who believe that they can coordnate devices when it comes to this issue. It like shooting at shadows unless you have a short time pickup and delay to provide something to work with. But as we both agree that comes at a cost and is it worth it.

 

[jim dungar]

Until an understanding and an appreciation is developed for how an OCPD responds to arcing and bolted faults there will be those who believe that they can coordnate devices when it comes to this issue.

They also need to understand that 'grounds' are the most likely type of fault and that they should be considered as part of coordination. There is stuff that needs to be considered other than just the extremes of long time overloads and bolted faults.

 

[techniphase]

Great feedback guys...Thankyou...so I went back and fired up all equipment. So happens we have a delta hi leg. 120/240 and c phase 208 nominal. Older system no adjustments on breaker. Crouse hinds .

A phase - 115 amps
B phase - 97 amps
C phase - 20 amps ( hi leg)

Although the balance is way off between phases, would his trip a main? It's a 200 amp and full tilt I'm getting
Say 235 amps but didn't trip this time...

Breaker probably shot ?

They have a small sub off the sub which is fed off a and b phases. Makes sense but when do you stop loading them up.......geeze.

 

[templdl]

Great feedback guys...Thankyou...so I went back and fired up all equipment. So happens we have a delta hi leg. 120/240 and c phase 208 nominal. Older system no adjustments on breaker. Crouse hinds .

A phase - 115 amps
B phase - 97 amps
C phase - 20 amps ( hi leg)

Although the balance is way off between phases, would his trip a main? It's a 200 amp and full tilt I'm getting
Say 235 amps but didn't trip this time...

Breaker probably shot ?

They have a small sub off the sub which is fed off a and b phases. Makes sense but when do you stop loading them up.......geeze.

Did the breaker trip thermally or instantaneously? Generally speaking if y ou are present at the time the breaker tripped if it tripped thermally because of an overload the breaker may be warm and you would have to let it set for a bit before you could latch it and reclose.
If it tripped instantaneously you could latch and reclose it immediately. But, it is a very bad practice to close a breaker back into a fault.
Also, check the load end connections to the breaker to assure they are tight and not adding heat to the breaker. If they are please be aware that the load end connects are very close to the thermal elements of the trip unit and will cause the thermal elements to derate resulting in the breaker nuisance tripping. Line side connections are not very likely to do this.

 

[Strathead]

Great feedback guys...Thankyou...so I went back and fired up all equipment. So happens we have a delta hi leg. 120/240 and c phase 208 nominal. Older system no adjustments on breaker. Crouse hinds .

A phase - 115 amps
B phase - 97 amps
C phase - 20 amps ( hi leg)

Although the balance is way off between phases, would his trip a main? It's a 200 amp and full tilt I'm getting
Say 235 amps but didn't trip this time...

Breaker probably shot ?

They have a small sub off the sub which is fed off a and b phases. Makes sense but when do you stop loading them up.......geeze.

Let me go a little in to how an overload trip operates... First Standard breaker tolerance is + - 10% This is significant of itself, that means a 200 amp breaker may trip as low as 180 amps or never trip as high as 220 amps. That said, an amperage in the call it "near trip setting" could potentially take hours or even days to overcome the intentional delay function of the breaker. Generally breakers have up to three different trip settings. They are referred to as long time (over 1 second), short time .1 to 1 second) and instantaneous (less than 5 cycles). Most commercial and smaller breakers only have long time and instantaneous. There are various methods of intentionally delaying the trip time.. electronic, thermal, hydraulic, pnuematic, geared escapement to name a few. For a 200 amp breaker such as you are using it likely only has long time and instantaneous and the long time is likely a thermal overload. Generally a spring holds the contact open and when a bimetal strip overcomes the opposition of the spring it operates the trip function (honestly, I can explain the actual mechanism of all the other functions more acurrately, but the thermal one is less capable of being "fixed" so we never really needed to "understand it so this may be a little simplistic). The instantaneous trip is generally operated via magnetism or electronic only and works 100% independent of the long time trip. This is part of the importance of all I am explaining. While the trip element for the long time is doing its thing, the instant element sits there, the instant that the amperage exceeds the value to overcome its opposition force, the mechanism operates and bam! So, to solve your problem, the first thing you need to do is determine which function is causing the trip and then go from there.


A couple things:
So an instant trip is going to require an intermittent huge spike in the amperage of about 2000A unless the instant trip element of one breaker is defective.

As mentioned earlier, a long time trip will heat the terminations at least, probably the case of the breaker depending on design.

You likely need a healthy 270 amps or more for a reasonable length of time (over a minute) to even think about tripping the long time. This should be easily detectable with a meter.

Not mentioned earlier, but each pole operates independently of the others, so imbalance doesn't create its own issues, but excess amperage in any one phase does.


I hope this long diatribe helps you to logically approach your troubleshooting, that was my intent.