To trip, or not to trip...

[kingpb]

MV metalclad switchgear with 125Vdc control power. Already monitoring the control power, and trip coil. If control power is lost; do I trip the breaker, or just alarm (DCS and local) and leave the breaker in it's current position?

I know distribution circuits typically just alarm cause the utility doesn't want customers in the dark, and Data centers, well, they don't want to trip for any reason, but this is not a sensitive process, or a facility that would be considered critical infrastructure.

My first impression would be to trip on loss, but what, if any is typical?

Cheers~

 

[ggunn]

Make sure you are in a safe and comfortable environment, no commitments where you have to deal with people for about 12 hours, plenty of orange juice on hand, something to occupy your mind like maybe some Grateful Dead music or Firesign Theatre, and... Oh, wait, you are talking about something else, aren't you?

 

[big john]

To be clear, are you asking whether you should put in DC UV or capacitive trips? Because if you lose control power you often lose DC tripping ability.

I don't see the concern about potential nuisance tripping of the customers equipment, because you should literally never lose DC trip power.

And I've seen the damage that can be inflicted because of the loss of DC during a fault, and it far, far outweighs any monetary concerns over nuisance tripping. So my opinion is if you have the option, make it fail-safe and trip on loss of DC.

I will end that with the caveat that most places I see do not go that far, and having a monitored DC source and battery backup is considered enough redundancy.

 

[Bugman1400]

If the loss of DC also means the loss of any relay protection, you are better off to trip for loss of DC. The exception to that is if your upstream protection is not on the same DC ckt and is sensitive enough to see the same fault.

 

[kingpb]

Thanks for the insight. I agree it is a very severe condition, but honestly had not given it much thought before. Always figured the dedicated battery system, with alarms and monitoring; was sufficient level of source voltage that it was not a concern. But with AF being such an issue, and SEL relays all operating off of DC power, wouldn't want to have to manual trip from in front of breaker.

I think I will have breakers be provided with an under voltage direct trip device which monitors the DC control voltage (one for each cubicle) that will mechanically trip the breaker if DC voltage is lost.

 

[qcroanoke]

Make sure you are in a safe and comfortable environment, no commitments where you have to deal with people for about 12 hours, plenty of orange juice on hand, something to occupy your mind like maybe some Grateful Dead music or Firesign Theatre, and... Oh, wait, you are talking about something else, aren't you?

Yeah, that's no fun if they find out........

 

[Tony S]

Easiest way is a DC UV trip coil on the breaker(s).

Most modern trip supply units come with the ability to supply a continuous auxiliary load. The older ones were a pain in the backside. I never did find out which muppet turned off a charger, the result, an 11KV board with no protection for a week.

We only fitted UV coils to the two incoming breakers, no need to fit them to the outgoing ways.