ungrounded wye secondary safety

[Paul Polakoff]

I am working on a drilling rig that is fed from a 4160 volt feeder. The transformers on the rig are delta 4160 primaries and wye 600 and 480 secondaries. The manufacturer chose to leave the xo to ground cables off, to allow for a ungrounded system.

My question is safety. The vfd's do not see a ground fault, nor do the breakers.
I have not found NEC to allow it other than for a load that is exclusively a vfd.

Is an ungrounded system safe? Is the potential less allowing the phase to run to the bonded equipment possibly for long periods, verse bonding the xo and having an immediate fault?

Paul Polakoff

 

[david luchini]

See NEC 250.20(B). If the 600V and 480V wye transformer secondaries do not use the neutral as a circuit conductor, then they can be ungrounded systems per the NEC. If the neutral is used as a circuit conductor, the system must be grounded.

 

[jeremysterling]

I'm not sure if a drilling rig falls under the scope of the NEC.

Some naval ships run ungrounded systems to be more battle-worthy. Perhaps the drilling rig engineers need reliability more than fault-clearing.

In 250.21, ungrounded system requirements are stated.

250.21(B) says all ungrounded systems shall have ground detectors.

As far as safety on your drilling rig, a monitored ground detector may be the solution.

 

[Paul Polakoff]

Thank You so far. I would feel better if the moniter was brought into the plc and displayed on the pc. But, I am just amazed that a fault can be allowed to go on for a long period of time.

I am concerned of a circulating current doing damage to bearings.

I am concerned that personnel could be harmed if time is allowed to pass.

I just have no lessons of hard knocks from using an ungrounded system, and wish to learn without an accident.

Paul Polakoff

 

[Paul Polakoff]

Mr Sterling

It should fall under NEC, because it was speced to fall under NEC. It may be that the drilling Industry likes ungrounded systems for no shut downs on faults, but we speced NEC compliance.

If I read the 250.21 it seems that the load exclusively has to be a vfd. The mcc loads are not entirely vfd.

Thank You
Paul

 

[jim dungar]

Most new generation VFD's (about 10-15 years) do not like ungrounded systems. They have 'filtering' capacitors that are connected to their chassis. You must follow manufacturers recommendations when applying them on ungrounded systems or they could fail violently damaging the VFD.

 

[Electric-Light]

Mines, refineries, shipboard, aviation etc use odd systems that are not used in standard commercial and industrial settings.

Ships have 60/120/173 high-leg delta. APC makes some special UPSs for shipboard use with dual pole everything inside for use on 60-0-60 configuration.

Some refineries and mines use 830Y/480v to reduce voltage drop on long wiring over standard 480v system while still allowing common 480v motors to be used by wiring them in wye.

high resistance ground is apparently used in some places so a fault between a leg and a ground won't down the operation while still providing some level of safety to prevent casing from becoming energized from high resistance leakage or capacitive coupling.

 

[Paul Polakoff]

I am still looking for replies about the safety to personnel differences between having a ungrounded system to a grounded system, pertaining to not grounded the xo on a wye secondary.

I hope someone has some experiance with ungrounded systems.

Thank You
Paul

 

[jim dungar]

I am still looking for replies about the safety to personnel differences between having a ungrounded system to a grounded system, pertaining to not grounded the xo on a wye secondary.

I hope someone has some experiance with ungrounded systems.

Thank You
Paul

Neither system is inherently better or worse for the safety of people.

Proper bonding of all non-current carrying surfaces is more of an safety issue for people.