Ground fault detection for separately derived systems

[dema]

The local electrical inspector is very concerned that 240V tanning beds inherently do not have ground fault protection. In delta services, ground fault indication is required. The tanning bed panel is a separately derived delta system and he would like to require ground fault indication on the panel serving the beds. It seems like this should be relatively easy and while not inexpensive, still not prohibitively costly to install. It seems like there should be a device with six CT's on it that could be installed, maybe on the secondary of the transformer if it was too big to go in the panel. Do you know of such a device? Have you had these safety concerns?

I've had so many weird questions that now the local inspector is calling me with them. YIKES!

 

[GoldDigger]

Is the transformer output an ungrounded or corner grounded delta?
If ungrounded, a ground detector must be provided. If grounded, then a ground fault protector may be desirable although not required.
The equipment itself may generate an undesirable amount of capacitive current to the shell of the bed if it is powered by either one grounded and one ungrounded or two ungrounded lines of a corner grounded delta. This could happen if the equipment was designed for and tested with a supply that has balanced 120V to ground on the two power leads.

Tapatalk!

 

[charlie b]

I don't understand. "Ground Fault Protection?" That phrase comes into play in 230.95, and speaks of maximum trip settings of 1200 amps. That is not going to protect any tanning customers. Does the inspector mean "ground fault circuit interruption"? That only applies to 120 volt systems. What is this person asking for, and how is it supposed to work?

 

[don_resqcapt19]

I assume he is really asking about the ground detectors that are required by 250.21(B).

 

[dema]

Dry type transformer

Dry type transformer

This is not a service transformer. It is just serving the delta loads - ungrounded delta loads. It isn't a service, so ground fault protection is not required. It is 240V and 400A, so it wouldn't be anyway.

So, is it your interpretation that ground fault detection is required according to NEC 250.21? What kind of expense is something like this?

 

[texie]

The local electrical inspector is very concerned that 240V tanning beds inherently do not have ground fault protection. In delta services, ground fault indication is required. The tanning bed panel is a separately derived delta system and he would like to require ground fault indication on the panel serving the beds. It seems like this should be relatively easy and while not inexpensive, still not prohibitively costly to install. It seems like there should be a device with six CT's on it that could be installed, maybe on the secondary of the transformer if it was too big to go in the panel. Do you know of such a device? Have you had these safety concerns?

I've had so many weird questions that now the local inspector is calling me with them. YIKES!

Is this separately derived system a 240/120 4 wire system or a 240, 3 phase, 3 wire? I'm not sure why one would ever use a 3 wire delta system (grounded or ungrounded) for this application. I'm not even sure it would be compliant.

 

[dema]

240 V delta

240 V delta

The building power is 208Y. The tanning beds use 240V delta. Three wire. If you have a motor, you run three wires and the equipment ground. That's what they've done here. They used a delta wye transformer backwards. The 208V wye is the primary. And grounded. The secondary is delta.

I suppose a bad enough ground fault would trip the transformer feeder breaker - but it would have to be pretty wicked.

I was asked to come in after this was built. Looking on the internet - this isn't that unusual. That doesn't make it right however.

 

[jim dungar]

The building power is 208Y. The tanning beds use 240V delta. Three wire. If you have a motor, you run three wires and the equipment ground. That's what they've done here. They used a delta wye transformer backwards. The 208V wye is the primary. And grounded. The secondary is delta.

I suppose a bad enough ground fault would trip the transformer feeder breaker - but it would have to be pretty wicked.

Absolutely do not have a connection to the X0 terminal of a 'reverse connected' transformer. The transformer should only be fed with the 3 phase conductors and an equipment grounding conductor.

According to the NEC, your 240V HV secondary is allowed to be corner grounded (i.e. grounded-B phase) or ungrounded with GF alarm/indication.
A lot of equipment (e.g. breaker panel) is not designed for ungrounded installations.

I would have a hard time believing that this facilitiy meets the NEC requirement of "conditions of maintenance and supervision ensure that only qualified persons service the installation".

 

[darekelec]

I did a research recently about delta corner grounded system and this paper is the best to understand the subject

http://static.schneider-electric.us...n/Molded Case Circuit Breakers/2700DB0202.pdf

Basically you need ocpd within 25 ft from xfmr on secondary and ground B phase to building grounding electrode system
or use ground fault detector.
If the secondary is not grounded, first ground fault will not trip ocpd.
I still did not conclude if fuses ore ok as protection on secondary. Anybody, please explain.

 

[GoldDigger]

I think that fuses with appropriate time characteristics are always OK. Just may be expensive to replace and also not touch safe for non electrician.
If this is the kind of customer who calls you out to reset a tripped breaker, no problem.

Tapatalk!

 

[don_resqcapt19]

...
I suppose a bad enough ground fault would trip the transformer feeder breaker - but it would have to be pretty wicked.
....

If the system is an ungrounded system, the first ground fault does nothing.

 

[jim dungar]

If the system is an ungrounded system, the first ground fault does nothing.

And the second one is easy to find, just look for the burn marks.

Ungrounded systems are not worth the headaches they will cause as they age.

 

[texie]

I still question if the tanning bed would be listed to be connected to a 240 volt 3 phase, 3 wire system system, corner grounded or ungrounded. It would seem that you would have to look at the standard it is listed to and what that standard says about power supply.

 

[kwired]

The building power is 208Y. The tanning beds use 240V delta. Three wire. If you have a motor, you run three wires and the equipment ground. That's what they've done here. They used a delta wye transformer backwards. The 208V wye is the primary. And grounded. The secondary is delta.

I suppose a bad enough ground fault would trip the transformer feeder breaker - but it would have to be pretty wicked.

I was asked to come in after this was built. Looking on the internet - this isn't that unusual. That doesn't make it right however.

But is any of the conductors of the separately derived system (the 240 volt delta secondary) grounded? Grounding conductor of the primary (208/120) system means nothing to the isolated secondary, as the primary and secondary windings are physically separate from one another until you ground a point on each side. I think you either need ground fault detection system - if you want to leave this as an "ungrounded" system, or you must ground one of the phase conductors since you likely have no secondary neutral to ground, to make the separately derived system a grounded system. Doing so may introduce other issues, like needing straight 240 volt rated circuit breakers, but if they are 3 pole breakers, they probably are already rated for this, but you would need to watch this for 1 or 2 pole breakers if using any. Breakers rated 120/240 can not be used on such a system.

 

[dema]

Grounding requirement

Grounding requirement

Where does it actually say that the 240V system serving loads with no neutrals has to be grounded? I thought that was a rule for anything other than industrial facilities. But I am not finding that to be true.

 

[david luchini]

Where does it actually say that the 240V system serving loads with no neutrals has to be grounded? I thought that was a rule for anything other than industrial facilities. But I am not finding that to be true.

Nothing says that a 240V, 3ph, 3w system has to be grounded. 240.21(B) says that if the system is ungrounded, it shall have ground detectors installed on the system.

 

[GoldDigger]

Also, if the system at that voltage has a neutral (even if the neutral does not serve any loads) then that neutral must be grounded.

Tapatalk!