Lost Phase

[Morpheus]

One of the posts in another thread made me think of this story, thought I'd share:
One of the spaces on our campus has an ETC 48 unit rack, and we're using multiplexers along the 4 FOH pipes (only stage lights, really)... the dimmer rack pretty much does all the lighting control.
Anyway, as part of the chapel program, I was one of the 'primary' users of the space.... also, in the most 'technologically advanced' user group... all the others expect it to work when they press all the architectural buttons neutral

Anyway, during one of our prolific holiday services apparently "some lights went out" and they wouldn't come back on. Of course, I didn't find out until I got back to school from christmas break, and went to go use the lights... and noticed a solid third of them weren't coming up... I ask the space custodians, and they say that about a 1/3 went out during a service, and didn't come back on.... so I head up to the rack... lo and behold, "PHASE C ERROR".
...
That's a mains issue, no way am I dealing with the mains service line.
Campus electrictian comes, apparently there is no three phase to the ceiling (or to the building, at that time), so they wired up a three wire single phase breaker box. Anyway, he opens up the box... apparently they blew one of the 400 amp fuses. So he returned a bit later, installed a new one, back to normal...

I was still kinda like wtf you lost 1/3 of your lighting, didn't you think it'd be a problem?

 

[SHARYNF]

Campus electrictian comes, apparently there is no three phase to the ceiling (or to the building, at that time), so they wired up a three wire single phase breaker box. Anyway, he opens up the box... apparently they blew one of the 400 amp fuses. So he returned a bit later, installed a new one, back to normal...

I was still kinda like wtf you lost 1/3 of your lighting, didn't you think it'd be a problem?

Can you explain further the three wire single phase breaker box vs the fuses?

Was this a three fuse disconnect? if it had three fuses it would be three phase since you never fuse the neutral?

I have seen this where the actual supply from the electric utility looses a phase say from a failure of a connection from the Pole typically.

A lot of the lower level maintenance types don't realize that you can blow one fuse and not the others, and sort of stupidly think if there is any power coming out and the breakers are all fine than there is no problem

At least you rack told you of the problem

Good thing you did not have any motors or AC/HVAC using the service with one leg out

Sharyn

 

[Morpheus]

Can you explain further the three wire single phase breaker box vs the fuses?

Was this a three fuse disconnect? if it had three fuses it would be three phase since you never fuse the neutral?

I have seen this where the actual supply from the electric utility looses a phase say from a failure of a connection from the Pole typically.

A lot of the lower level maintenance types don't realize that you can blow one fuse and not the others, and sort of stupidly think if there is any power coming out and the breakers are all fine than there is no problem

At least you rack told you of the problem

Good thing you did not have any motors or AC/HVAC using the service with one leg out

Sharyn

Err... tbh, I'm not really sure I understand your question, or the wireing in question - nor am I going to claim too. I am fully ok with leaving this part to a licensed electrician

What I do know is what I've been told -
Apparently the building (it was built in the 1920's) only had a single phase distro when the system was put in...
So, to accomodate the ETC rack needing three phases, they put in a box, and all I know is that it somehow splits the single leg into the three necessary tie ins for the dimmer rack, and that they have fuses... which somehow one of them blew.

Three phase disconnect sounds like it could be right.
Yea, I don't really want to be dealing with the mains power, especially if I don't have too, so that's why I don't know all that much about it... I trust our campus electricians, if they say it's good, I'll go with it.

If i need to know this stuff in the future, I'll learn it right before I try it.

And this is solely for our rack, it's all located in the ceiling of the space. (rack and disconnect)

 

[SHARYNF]

Sounds like fun fun fun

If that is what they did basically feed two hot hot legs and neutral in and then split one of the hot legs, the issue typically would be the need for a higher rated neutral, all of which would be causing problems

Typically when a main fuse like you are talking about blows there is a major problem that someone needs to at least take a look at and see what is going on Certainly something NOT for you but the electrician. There is lots of confusion on electricians part that have not worked with the sort of loads that Dimmers present on how the neutral is being sized. in essence if what you are saying is correct, then one supply leg is really at 400 amps and the other is at 800 amps OR they simply doubled the load and you have one supply at 400 amps and then the other two feeds to your dimmer rack THINK they are 400 amps each but in fact you are presenting a possible 400 amp load on each feed all from a 400 amp breaker


This could be the root of your problem where you THINK you have 400 amps on each feeder to the dimmer rack, when in fact you don't

Could be that now you are using more channels on your dimmer system

If you do not want to have this problem again you might want to have the electrician look at this carefully

Sharyn

 

[Morpheus]

All right, thanks.

I have a feeling that this was just a freak occurrence (it happend 2-ish years ago, and no other time to my knowledge) - the system designer is one of my current theatre profs (resident light & sound designer), one of the other main users is the TD/facilities manager of the other campus theatre...

But it certainly wouldn't hurt to double check. I'll pass this info along to the relevant people... if needed, i could probably ask the electrician myself next time i see him in passing.

 

[SHARYNF]

I'd be interested to see what comes up

I would GUESS that someone might have just only used two 400 amp fuses in a disconnect and then just run two of the phase inputs from on of these feeds and the other one from the other 400 amp, So in a lightly used situation, probably never saw the problem but If for instance the house lights AND the two legs happen to ALL be on that doubled feed, that is what could have blown the fuse Someone probably assumed that the house lights AND the theater lights might not be all on at the same time

Sharyn

 

[Morpheus]

No, I'm pretty sure they designed it to be used with the stage and house lights at the same time... it's designed to be fairly idiot proof...

However, they DO on occasion bring in some big lights for said holiday event...
It may be possible that they put too many on and blew a fuse that way...
The specified holiday event is run by a separate part of the college, not related to what I was involved in.

 

[jared555]

The only thing I can think of that would be a somewhat safe way they could have wired the system is that instead of bringing in three phase power they ran three 120V lines that were all the same phase. This would be safe (not necessarily for the equipment, shouldn't cause a fire though) to do as long as the neutral/ground could handle all the lines combined (which sounds like it would be 1200 amps peak)

Although with the cost of running that thick of wire for the neutral they probably could have ran 3 phase. I don't know if even 0000 AWG cable would be enough for that kind of load.

 

[photoatdv]

I think where they need a thinker wire they can run 2... hence the 2 neural ins/outs on some distros.

 

[tjrobb]

Photo - You're thinking of parallel wiring and are correct, over 1/0 you are allowed to run multiple smaller runs of cable to equal the load (there are several rules for this however). This is common where the larger cable is too unwieldy.
Other than parallel wiring / oversize cable double neutrals are used where the phase load is far enough out of balance to cause excessive neutral current.