grounded B phase. newbie please help

[cmaki]

I am a new maintenance tech at my work. I am the only tech and no one else seems to know anything about electrical stuff here which is a good thing. We have 480V and what I believe is 480 grounded B as well. I know nothing about the grounded B system. It is labeled at one point as 240V. When I check it I get 240 to ground on A and C and nothing on B. Phase to phase is 480. Is this system 480 or 240?
Sorry about this but I have a bunch of questions. Can this system be ran through disconnects labeled for 250VAC? I see on at least 1 system the B phase is fused. Is this allowed? I thought I read somewhere that the B phase needed to be unfused. Please advise.
I have 480 3ph coming from a panel going to a transformer where it comes out grounded B. Is there any purpose for this? Why would we not just keep running the 480 3ph?
I found 1 disconnect on the grounded B phase that is fused at 30amps. It then goes into an outlet and carries on to another disconnect with 40 amp fuses. This seems completely backwards. The second disconnect was added recently for a new paint booth. (I guess the last tech was a real idiot)

Thanks for your help

 

[augie47]

I am a new maintenance tech at my work. I am the only tech and no one else seems to know anything about electrical stuff here which is a good thing. We have 480V and what I believe is 480 grounded B as well. I know nothing about the grounded B system. It is labeled at one point as 240V. When I check it I get 240 to ground on A and C and nothing on B. Phase to phase is 480. Is this system 480 or 240?

The numbers don't make sense. It sounds more like you have a 480volt with B phase grounded but your A & C would measure 480 to ground.

Sorry about this but I have a bunch of questions. Can this system be ran through disconnects labeled for 250VAC?

If it's 480 phase to phase, No.

I see on at least 1 system the B phase is fused. Is this allowed? I thought I read somewhere that the B phase needed to be unfused. Please advise.

B Phase would not be fused on a system where B phase is grounded.

I have 480 3ph coming from a panel going to a transformer where it comes out grounded B. Is there any purpose for this? Why would we not just keep running the 480 3ph?

Most likely the transformer has an output other than 480v

I found 1 disconnect on the grounded B phase that is fused at 30amps. It then goes into an outlet and carries on to another disconnect with 40 amp fuses. This seems completely backwards. The second disconnect was added recently for a new paint booth. (I guess the last tech was a real idiot)
Thanks for your help

Not necessarily a problem..the second disconnect may be being used as a disconnect means and not overcurrent protection.

From the nature of your questions I would urge you to arrange for a qualified electrician to review the facility electrical systems with you with the goal that the systems are properly identified and Code compliant. There appear to some potential problems.

 

[winnie]

As a maintenance tech who does not understand electrical theory, make sure you don't get pushed by your employer to do electrical work that you are not qualified for.

Voltage is always a measurement between two points. In a three phase 480V system, you will have 480V from phase A to phase B, 480V from phase B to phase C, and 480V from phase C to phase A. The 480V between phases tells you nothing about the voltage from phase to ground.

You can connect _any_ of the transformer terminals to ground, and use that terminal as your ground reference and bonding point. The ground connection does not change the voltage between any two terminals; it simply defines the voltage between all of the terminals and ground.

As long as you don't have connections between _two_ terminals and ground, you don't have a short circuit.

In a very common system, the 480V comes from a 'wye' transformer. The voltage from any phase to that central point will be 277V. That center point gets connected to ground, and because of this connection, if you measure from phase to ground, you will get 277V.

However you can just as easily connect one of the phases to ground. If you do this, then the voltage of that phase _to ground_ would be forced to be zero by the direct connection. This direct connection does not change the phase to phase voltage, only the phase to ground voltage. If you take a 480V system and ground the B phase, then you get:
Phase A to phase B: 480V
Phase A to ground: 480V
Phase B to phase C: 480V
Phase B to ground: 0V
Phase C to phase A: 480V
Phase C to ground: 480V

In general, there should either be no fuse on the 'grounded conductor', or you should have overcurrent protection that disconnects _all_ of the conductors. This is usually done with common trip circuit breakers, but I believe there are ways to set up 'common trip' fuses.

The measurements that you describe are not consistent with a three phase 'grounded B' system. It looks more like a single phase 'center tap' 480V system. In such a system you have:
Phase A to Phase 'B' 480V
Phase A to neutral: 240V
Phase B to neutral: 240V
neutral grounded at 0V to ground

If you actually have 480V between the two 'phases', then a 250V rated disconnect is not suitable.

A 240V grounded B system would have the following voltages:
Phase A to phase B: 240V
Phase A to ground: 240V
Phase B to phase C: 240V
Phase B to ground: 0V
Phase C to phase A: 240V
Phase C to ground: 240V

A 250V rated disconnect could be used for a 240V grounded B system, but watch out; some devices are 'slash' rated for 125/250V, which means that the device is rate for 250V phase to phase, but only 125V phase to ground.

If the system entering the building is a 480V grounded B system, and some load needed 480V single phase neutral ground, then a transformer can be used to create a new set of 'isolated' terminals. You can ground the new terminals any way you like, without worrying about how the source is grounded.

-Jon

 

[cmaki]

Sorry I feel like an idiot now. Its 240 phase to phase and 240 to ground. not sure where I got the 480 from.

It at least 2 spots now I have found the phase to ground with a fuse.

 

[augie47]

Sorry I feel like an idiot now. Its 240 phase to phase and 240 to ground. not sure where I got the 480 from.

It at least 2 spots now I have found the phase to ground with a fuse.

That makes more sense (and improves your creditability :roll. There would be no problem with a 250 v fusible disconnect but B phase should not be fused. They make "dummy fuses" to replace the real fuse as one possible solution.
The same is not true for panelboards. A grounded phase system requires particular panelboards and breakers (see 240.85 in the Code if you ae interested in the particulars)

 

[cmaki]

Can someone explain to me (and then my boss) why we need to remove the fuses and put in dummy fuses? I kinda understand that the grounded leg is hot and the ground at the same time. I need a good reason to spend money on these.

 

[GoldDigger]

Can someone explain to me (and then my boss) why we need to remove the fuses and put in dummy fuses? I kinda understand that the grounded leg is hot and the ground at the same time. I need a good reason to spend money on these.

Very very simple:

1. You have grounded the B phase on the POCO side of the fuse.
2. People will get used to the fact that the B phase is at zero volts and will get careless about touching it.
3. If the fuse on the B phase opens and the fuses on either or both of the other phases do not open, then there will be full phase voltage on the B phase wires downstream of the fuse. (Coming through the connected loads.)
4. Somebody will get hurt or killed touching what they think is a grounded wire.

And, no the grounded leg is not hot and ground at the same time. It is current carrying at all times that there is a load connected though. And that is why interrupting it can change the voltage on it downstream.

 

[ActionDave]

If you can understand how you can be shocked by an open neutral on a household circuit then you can see the danger.

If that B phase fuse opens up everything connected to that is grounded gets its voltage potential raised to 240V.

 

[cmaki]

If the b-phase is grounded before the fuses and there is no ground after the fuses either how will voltage get to ground if the b-phase fuse opens. that is where I am lost.

Im not an electrican but a jack of all trades maintenance guy. No real formal training but a lot of on the job training. This 240V CGD stuff is new to me

 

[GoldDigger]

If the b-phase is grounded before the fuses and there is no ground after the fuses either how will voltage get to ground if the b-phase fuse opens. that is where I am lost.

Im not an electrican but a jack of all trades maintenance guy. No real formal training but a lot of on the job training. This 240V CGD stuff is new to me

The voltage will not get to ground (or to the source wire at the service) if the fuse opens, but the voltage will still be on the wire and capacitive effects to ground, if nothing else, can still cause injury.
And the secondary connectors at the supply transformer, will still be ground referenced by the bond upstream of the fuse. That means that if you form a bridge from an ungrounded conductor to ground the current will get back to the transformer secondary through you!

Stating it a different way: Opening the fuse in the grounded conductor may stop the equipment from operating, but it will not remove dangerous voltages from wires that used to be at ground.

 

[ActionDave]

If the b-phase is grounded before the fuses and there is no ground after the fuses either how will voltage get to ground if the b-phase fuse opens. that is where I am lost.

Im not an electrican but a jack of all trades maintenance guy. No real formal training but a lot of on the job training. This 240V CGD stuff is new to me

It's not getting around anything. With the fuse intact you have voltage at ground potential and x amount of current flowing in the circuit. With the fuse open you have 240V above ground on everything that was grounded and no current flowing until you complete the circuit some how.

Golddigger wins again.