Racking out breakers

[Waldo84]

I have an interview soon for a maintenance company on a drilling rig and 1 of the questions they asks is
when racking out a circuit breaker you hear crackling/arcing do you keep racking out or rack it back in?

 

[LeeH]

The only thing I can think of is winding in and charging an ACB but I've never come across the term personally.

 

[Bobster]

Is this a HV circuit breaker. OCB/VCB etc...

 

[DPG]

Check out the youtube videos of what happens when you pick the wrong option! Daz

 

[Waldo84]

I've watched the YouTube videos and seen what happens. That's why me personally would rack the breaker back in because if it's arcing to must still be under load. Not a good idea to do so. I don't think the breakers are HV is 3.3kv or higher because the drill equip on the platform is all 600v DC
thank for the help

 

[Pete999]

The only thing I can think of is winding in and charging an ACB but I've never come across the term personally.

The breaker should be locked off and earthed prior to racking it down, so it
if it's an OCB the the oil tank you are racking down it should be isolated, I can't help you with VCB as oil is my only experience, so I think this may be a trick question, as the interlocks will prevent you from racking anything out of circuit, it's been a long time so my answers may be a little dated

 

[7029 dave]

As above, the way I take their question, my answer would be back in.

 

[LeeH]

The breaker should be locked off and earthed prior to racking it down, so it
if it's an OCB the the oil tank you are racking down it should be isolated, I can't help you with VCB as oil is my only experience, so I think this may be a trick question, as the interlocks will prevent you from racking anything out of circuit, it's been a long time so my answers may be a little dated

Ah ok, so what they are implying if you are 'racking out' as they call it and you hear arcing it's still live.

You obviously wind it back in again. I've not done a huge amount work with ACB's but I'm sure the Merlins don't let you wind it out if it's still charged??

I'm sure one of the chaps on the heavy side of the game will know for sure.

 

[Darkwood]

Run!!!!!!!!.... fast and dont look back

 

[Pete999]

https://www.youtube.com/watch?v=fuCX-PgEDrk

 

[londonlec]

Could someone explain (to someone with no experience over 400v) what racking in and out breakers actually means? Many thanks guys

 

[DPG]

I'm no expert but I believe it to be the process of removing and re-installing high voltage circuit breakers (for maintenance etc) which are made as separate modular assemblies. Someone who knows more than me will clarify this hopefully! daz

 

[bigdavs83]

The term racking out as far as I have came across is the air circuit breakers on 3 phase panels (415v). These are the ones with the charging spring and the red off and green on buttons on the front (dorman smith). These acb's feed all the isolators on there respective panel and this is in turn fed from the 11kv transformer. The process of racking it out ensures that there is no way of the contacts making if you are carrying out maintenance on either the 415 side or the 11kv side. The 11kv side may receive back feeds if there is a generator or ups tied into the system so the HV guys insist it is load shed before they carry out the works.

 

[cossie400]

Air Breakers, "winding" or "racking" in/out is the term used for when you remove the breaker from its cradle.
you literally insert a handle and turn it to remove or insert. Some of the older ones you literally pumped the handle up and down instead of winding.

*edit Bigdavs83 beat me to it!

 

[Knobhead]

OK now we’ve had the stupid scare stories and the shock video. Both RoB2 and I have both been SAP.

It isn’t arcing as such. If it was you wouldn’t be worrying about it, you would be dead. What you will hear is a hissing sound, capacitive charging of the circuit breaker contacts. The higher the system voltage the sooner it starts. 3.3KV you hardly notice it, at 11KV it will start about 1” before the isolating contacts touch.

So what do you do when you hear the hiss? Don’t stop winding the racking gear is the answer.

Most breakers can’t be racked in or out unless they are open (OFF). That applies to both LV and MV (HV in the UK).

BTW, you won’t be allowed to carry out any MV duties until after training.