480 delta grounding

[texie]

I have a situation where the EC installed a step up 240 delta to 480 delta transformer to feed a crane. At present the 480 is not grounded. Of course it can be corner grounded, but brings on other issues such as the 3 pole fusible disconnect that is installed.
Question: If they corner ground does not the equipment have to be listed for corner grounding? I also wonder if the crane mfg. has any preference for supply to be Y grounded or delta corner grounded.

 

[GoldDigger]

I have a situation where the EC installed a step up 240 delta to 480 delta transformer to feed a crane. At present the 480 is not grounded. Of course it can be corner grounded, but brings on other issues such as the 3 pole fusible disconnect that is installed.
Question: If they corner ground does not the equipment have to be listed for corner grounding? I also wonder if the crane mfg. has any preference for supply to be Y grounded or delta corner grounded.

Maybe because it affects the insulation rating (phase to ground) required on the motors and other internal wiring of their crane, including other wires running in the same raceways as the phase wires? Or induced voltages on their control wiring?

 

[templdl]

I have a situation where the EC installed a step up 240 delta to 480 delta transformer to feed a crane. At present the 480 is not grounded. Of course it can be corner grounded, but brings on other issues such as the 3 pole fusible disconnect that is installed.
Question: If they corner ground does not the equipment have to be listed for corner grounding? I also wonder if the crane mfg. has any preference for supply to be Y grounded or delta corner grounded.

This is the way I would consider it:
You can not fuse the grounded phase conductor. Simply install a solid link in that phase and you are good to go.
You suggestion of using a "Y" is a good one though. If you don't absolutely need a delta go with the Wye. Some people believe that if their only need is a 480v 3ph that doesn't require any L-N loads they automatically specify 3ph3w which equals a delta in their minds which is not necessarily so. You can specify a wye and not bring out the X0 but just ground it.
By specifying a delta and corner grounding it in this case you will have 480v L-Grd. Shound you elect to use a Wye and ground the X0 as is commonly done you will end up with 277v-Grd which is more desirable. Also you may or may save a few bucks on the OCPD as a delta would require a 480v rated device and a wye would require a 277/480v device w2hich may or maynot be significant.

 

[texie]

I'm having an issue with this install for all the reasons you mention. Problem is, the EC claims thet have hundreds like this (480 ungrounded, no detectors). I'm saying, well maybe you have, but it is not code compliant unless you have ground detectors or corner ground the delta supply to the crane (and show evidence that that mfgr. of the crane approves and all equipment is fully rated for 480 volt.
This raises another question for me. How much equipment is installed out there on a corner grounded supply that is not rated for it?

 

[hurk27]

Most cranes I have worked on are feed with a 480 ungrounded delta, this is because loosing power while making a lift or lowering a load can be very dangerous if power were to fail I'm the middle of doing it if it has a mag lift power is even more critical, I know for charge cranes for furnaces this is a must as having a crane stop with the charge bucket in the furnace and they if can't get the doors of the furnace closed can be very costly, also cranes located over certain combustible fibers or dust are not even allowed to be grounded (250.22(1).

With that said A ground monitor is a must to let some one know a conductor has gone to ground.

 

[texie]

Most cranes I have worked on are feed with a 480 ungrounded delta, this is because loosing power while making a lift or lowering a load can be very dangerous if power were to fail I'm the middle of doing it if it has a mag lift power is even more critical, I know for charge cranes for furnaces this is a must as having a crane stop with the charge bucket in the furnace and they if can't get the doors of the furnace closed can be very costly, also cranes located over certain combustible fibers or dust are not even allowed to be grounded (250.22(1).

With that said A ground monitor is a must to let some one know a conductor has gone to ground.

Agreed. But that still means my install is not compliant.

 

[hurk27]

Agreed. But that still means my install is not compliant.

I agree and a ground detector needs to be installed, I'm not sure when the ground detectors were first required but even my 1999 copy has it in 250-21(3)(d) but it was only for separately derived systems, so it might have been in the past that Ground detectors were not being used if the supply was an ungrounded system from the utility?

I know I would want them as I would want to know when a conductor went to ground because the next conductor that goes to ground will cause a fault and shut down the equipment which is why you want a ungrounded system in the first place.

I wouldn't care if they have done hundreds of installs with out Ground detectors, all that says is they have been doing it wrong for hundreds of times and that is bad.

Show them 250.21(B)

(B) Ground Detectors. Ground detectors shall be installed
in accordance with 250.21(B)(1) and (B)(2).
(1) Ungrounded alternating current systems as permitted in
250.21(A)(1) through (A)(4) operating at not less than
120 volts and not exceeding 1000 volts shall have
ground detectors installed on the system.
(2) The ground detection sensing equipment shall be connected
as close as practicable to where the system receives
its supply.

Ground detectors are used to provide a visual indication, an
audible signal, or both, to alert system operators and maintainers
of a ground-fault condition in the electrical system.
With notification of the ground-fault condition, rather than
automatic interruption of the circuit, the operators of the
process supplied by the ungrounded system can then take
the necessary steps to effect an orderly shutdown, determine
where the ground fault is located in the system, and can
safely perform the necessary repair

The above is from the 2011 with the commentary.

It has not changed much from the 2002 code cycle.

I agree that a crane is a good reason to have an ungrounded system but an ungrounded system without ground detectors totally defeats the reason for having an ungrounded system since you will not know when a phase went to ground until the second phase goes to ground then its to late.

Are you the inspector or is this work at a plant you work at? If the latter then I would show your boss (if it is appropriate) the above code and explain to him the reason I posted above.

 

[templdl]

Most cranes I have worked on are feed with a 480 ungrounded delta, this is because loosing power while making a lift or lowering a load can be very dangerous if power were to fail I'm the middle of doing it if it has a mag lift power is even more critical, I know for charge cranes for furnaces this is a must as having a crane stop with the charge bucket in the furnace and they if can't get the doors of the furnace closed can be very costly, also cranes located over certain combustible fibers or dust are not even allowed to be grounded (250.22(1).

With that said A ground monitor is a must to let some one know a conductor has gone to ground.

I Agree also. That's one of the few advantages of an ungrounded delta.