CB electronic trip unit

[powerjunx]

Anyone knows how to specify a power circuit breaker having electronic trip unit? a complete specification would be better.

Does Ground fault tripping is required for a minimum specification or it depends to the customer's discretion?

 

[DanDel]

Typical electronic trip functions are Long Time (LT), Short Time (ST), Instantaneous (Inst), and Ground Fault (GF).
LT is for overload conditions and is always necessary.
ST is for high current overloads or low level faults, and will provide a short time delay for these issues.
Inst is for short circuits, and GF is obvious.

Typically, a Main or Tie C/B would have LT, ST, and GF. You wouldn't want Inst, because this may trip along with a feeder C/B Inst, and cause a much larger outage. Per the NEC (if it applies to you), a Main C/B for a 480V (and above) system must have GF.

A feeder C/B should typically have LT, ST, Inst, and GF. This allows proper downstream protection and coordination with the upstream Tie and Main C/Bs. The NEC requires only feeder C/Bs of 1200A trip to have GF, but you'll find that specifying every C/B is typical and will help with GF coordination.

 

[davidbeach]

What DanDel says about not having INST on a Main or Tie is what has been done for many years and is very common. But, now in the day of Arc Flash hazard awareness, that may be changing. Without INST, you will not get fast clearing, - and the associated lower PPE requirements - unless you use additional relaying, such as bus differential relaying, for faults in the gear. You need to look at the whole picture.

 

[oldfieldguy]

Some users are applying a two-tier approach to protection in response to the arc-flash hazard: setpoint file 1 has the normal LT/ST curves that make sense from a time/current coordination standpoint, and serpoint file 2 which is MUCH faster, to reduce exposures to arc-flash hazard, the two being selectable by the user.

For normal operation, the protection runs with setpoint file 1. If switching and racking operations are going to take place, the user selects the faster setpoint file 2.

Of course, this requires a more advanced protection relay, available for a nominal fee...

old field guy

 

[dpc]

The major manufacturers should be able to provide you a sample specification. If you compare two or three specs you should be able to weed out the blatantly proprietary stuff.

Newer breakers can be purchased with switchable instantanteous and ground fault, so that these can be switched off if not desired for a particular breaker.

 

[rbulsara]

With all due respect:

"Anyone knows how to specify a power circuit breaker having electronic trip unit? a complete specification would be better."

Sounds like you want someone else to do the job for you. Did you check with any manufacturers?

 

[alehman]

Oldfieldguy's idea has recently been implemented by some low voltage breaker manufacturers as a standard option.

 

[powerjunx]

rbulsara, i got your point. In my case, i'm on the verge of tracking and somewhat upgrade our maintenance program for our equipments due to abscence of data and limited operating manuals, i resort to ask such absurd query. The fact, i have limited knowledge about this trip unit.

Though these equipments niether experience trip out ,eversince. I'm optimistic that this will trip out, sooner.

Guys, i'm interested on Ground fault. I recently read a document which says "GF is 20-30% of the phase setting
and it's time setting is about or at least 0.1sec." How to discriminate this setting against INST setting? Anyone can ellaborate this, please?

Definitely, I 'm talking about a low voltage Circuit Breaker: Cutler-Hammer, DS II, digitrip 510 rms.

Unfortunately, i found hard to download this stuff on-line. Unsurprisingly, i post this inquiry.



Thanks a lot, guys!



bil

 

[oldfieldguy]

A lot of low voltage breaker trip units have a "ground fault" setting which is a drevied funciton based on current balance of the three phase sensor (CT) units. In this case, your guideline of 20-30% of phase pickup would probably be correct.

Other units actually require a separate outboard CT or sensor. These can be set at a more sensitive level, but add complexity to the installation.

Since the G/F setting is lower (MUCH lower) than any instantaneous setting, the G/F will operate for low order ground faults well before current reaches a level to operate any instantaneous element.

If the fault is high enough to operate both the G/F and the instantaneous elements, I want that fault cleared, regardless of what element takes the breaker open.

old field guy

 

[rbulsara]

No problem, fbcybil.

I am not sure you got answer to your original question. If I understood you correctly, you have to specify which functions you want in a trip unit. Essentially you need to pick one of the follwing sets, in your specification:

LI
LSI
LS
LSG
LSIG

You can also replace G with a A if you want groud fault "Alarm" only. G means it is for tripping on ground fault. There may be some other combination. LSI and LSIG are most common.

Now whether or not a ground fault proteciton is required by Code is a different issue. NEC spells out when a GFP is required. There may be other reasons to have or not have GFP. In any case you can not circumvent the code.

If you are looking for selectivity between a main and feeder breakers when a ground on a feeder occurs, you may want to have GFP in mains and all feeder breakers. Turst me this decision making takes lot of factors into account, and not possible to advise as a rule of thumb.

 

[rbulsara]

This may cause you more stress, but you can not defeat (choose not to use it) a GF function in a breaker trip unit in field. I beleive only one mfr. (Cutler Hammer??) offers that ability, you need to check. So you need to specify it correctly.

Replacing a trip unit in field is usually expensive and cumbersome.

 

[arcflash99]

We have been retrofitting trip units in the field for the last couple of years. After reviewing several mfr's units we decided to AC PRO trip unit manuf by utilityrelay. Since they have pick ups and trip times that are adjustable, they coordinate much easier than other trip units.
The new line from them has "Quick Trip" settings that basically like others have stated ther are an additional set of certain settings that are activated by the flip of a switch.
If anyone is considering using a relay that has multiple/ selectable settings be sure to have some sort of alarming capability so no one forgets to turn the switch back to "normal" operation.

My 2 cents