TRIP UNITS

[puckman]

I ask afew questions on the digatrip relays and got great information here and i am hopeing to get the same with this question.
Do the old amptector 1 relays need the same 20 % load on them before the trips will work ?

 

[zog]

Um, you won't have any trips at 20% loading. If a fault occurs the current will be high enough to power the trip units to trip the breaker.

But to answer your question, sort of, yes, same deal with Amptectors, nothing functional at less than 20% load, actually migh tbe closer to 30% for those. It is a matter of the CT's really, no output below a certian load. That is one nice thing about AC PRO's, the CT's they use still provide a usable output at 10% loading, some testeing shows closer to 5%, you don't ever have to change CT's or rating pugs for different loads and all the functions can be enabled or disabled for whatever your needs are. All simple things to do, no idea why the OEM's don;t design all trip units that way (Actually I do know, just don't want to be the one to say it)

 

[puckman]

This is what i was looking for and it appears to have solved a unanswered question . Thank you zog.

 

[hawkeye23]

So if a 500 amp. circuit breaker has 100 amps. on it and a fault occurs this circuit breaker will not trip ? Scary.

 

[zog]

So if a 500 amp. circuit breaker has 100 amps. on it and a fault occurs this circuit breaker will not trip ? Scary.

No, if a fault occurs the current will be more than 100A.

 

[hawkeye23]

In post 2 you say at 20% you won't have any trips. Then , if a fault occurs you will have enough current to power the trip unit to trip a breaker.
I am sure it is me but it sounds like it works either way. What doesn't work at 20% and lower ?

 

[rcwilson]

The electronic trip units are powered by the CT's. If the load current is too low, there is not enough current flow to power the logic. A 600 A trip unit with a 600A CT and an overcurrent setting of 0.15 pickup, should start thinking about tripping at 0.15 x 600 = 90 Amps. But 90 Amps is not enough current to charge up the electronics. When a fault occurs, the fault current will supply enough energy to power up the electronics and intiate a trip.

This is a poor example. A 600A unit would not have a pickup of 0.15. But it might have a ground fault settign of 0.15.

 

[zog]

In post 2 you say at 20% you won't have any trips. Then , if a fault occurs you will have enough current to power the trip unit to trip a breaker.
I am sure it is me but it sounds like it works either way. What doesn't work at 20% and lower ?

In post 2 I was saying the breaker should not be needed to trip if there is only 20% load. If you have a 3000A breaker why would you want it to trip at 600A?? If a fault were to occur the current would increase above that 20% mark (And much higher) so the trip unit would trip the breaker. Thought my points on both posts were clear, sorry if they were not.

CT's need to reach saturation before they have any output, for some older designs that is around 20%, newer designs are more like 10%. The CT's power the trip units, if they have no output they won't power the trip units, hence the minimum pick up setting of 0.5x for most trip units. Some trip units have battery backups in them but most plants never check them and they are often dead anyways. If the trip unit is not powered up you will not be able to read trip event history, see any metering functions, etc....so after a fault occurs when you go to the breaker to see why it tripped you won't see any data if the battery is dead.

 

[hawkeye23]

I Got it now . I though you ment at below20% the units did not work at all and if there was a fault of some kind like a ground fault the trip unit did not work . I now understand that leds are the only thing that would not work and if the battery is good pressing the test button would show the fault by lighting the led.

Thanks for the trip unit expirenance it will help us with the micro versa trip we have that so far is working good.

 

[zog]

Most trip units sense ground faults by comparing phase currents. So a GFPU of 0.15 on a 1600A breaker would trip on a difference of 240A between any 2 phases. Unbalanced loads can cause GF trips to occur. You can have a 4 wire system where the trip unit gets a signal from a 4th CT located in the switchgear for its GF input.