277/480 - single phase 480 lightning issue

[bobl]

I have run into a situation where a system that has already been installed is now failing due to lighting strikes. The local utility provides a pad mounted 120/240 transformer (3 phase) and locally there is a 3 phase generator (120/240) connected to an ATS.

There is a need for 480 volt single phase and 277 for fluorescent lighting from the system so a 3 phase 120/240 to 277/480 volt transformer was installed. The 277 volt lights are phase to neural and work fine. (Until a lightning strike occurs in the area) but to supply 480 volts to exterior lighting, phase to phase and a grounding conductor is used to feed light poles.

There is no neutral return path to the distribution panel and the breakers do not trip on the surges on the lines. TVSS devices are installed on the system and sometimes handle the surge but more often do not. In certain situations, the 277 lights on the building are knocked out due to the surges on the phases that are not protected.

I understand that this in not correct since there is no method to return a neutral to allow the breakers to trip.

I am trying to determine the code that would apply here that is possibly violated. (NEC 2005) Since this is a separately derived system as defined in Section 100, and is similar to Exhibit 250.7 in the NEC 2005 Handbook the apparent problem is the system is using 2 legs of the secondary of the transformer to feed a single phase lighting system.

I believe the method of remedying the situation and conforming to code would be to install a single phase 240 to 480 volt transformer in addition to the existing 277/480 volt transformer to feed the exterior 480 volt lights.

Any help on identifying code, verify the thinking on this situation as well as the comment on the possible solution would be greatly appreciated.

 

[zog]

I have run into a situation where a system that has already been installed is now failing due to lighting strikes. The local utility provides a pad mounted 120/240 transformer (3 phase) and locally there is a 3 phase generator (120/240) connected to an ATS.

There is a need for 480 volt single phase and 277 for fluorescent lighting from the system so a 3 phase 120/240 to 277/480 volt transformer was installed. The 277 volt lights are phase to neural and work fine. (Until a lightning strike occurs in the area) but to supply 480 volts to exterior lighting, phase to phase and a grounding conductor is used to feed light poles.

There is no neutral return path to the distribution panel and the breakers do not trip on the surges on the lines. TVSS devices are installed on the system and sometimes handle the surge but more often do not. In certain situations, the 277 lights on the building are knocked out due to the surges on the phases that are not protected.

I understand that this in not correct since there is no method to return a neutral to allow the breakers to trip.

I am trying to determine the code that would apply here that is possibly violated. (NEC 2005) Since this is a separately derived system as defined in Section 100, and is similar to Exhibit 250.7 in the NEC 2005 Handbook the apparent problem is the system is using 2 legs of the secondary of the transformer to feed a single phase lighting system.

I believe the method of remedying the situation and conforming to code would be to install a single phase 240 to 480 volt transformer in addition to the existing 277/480 volt transformer to feed the exterior 480 volt lights.

Any help on identifying code, verify the thinking on this situation as well as the comment on the possible solution would be greatly appreciated.

I have seen these systems before and typically there is a ground rod at each light pole.

 

[bobl]

277/480

277/480

There is a ground rod at each pole that is connected to the pole structure. Still there is no neutral. Is there issue with this type of system?

 

[wirenut1980]

Wow, a lot going on here...lots of questions.

Where is the TVSS installed? At the 480 panel feeding the lighting? What modes does it protect? Line to line, line to neutral? What is the initial clamping voltage?

Are you sure that the lightning is entering in through the 480 circuit feeding the exterior lights? How do you know?

How is the 120/240 -480/277 V transformer grounded? Is the transformer specially made for stepping up voltage or is it "reverse wired?"

One misconception is that a circuit breaker will protect equipment from a lightning strike or voltage surges.

The exterior lighting circuit you described with 2 hots and a ground is common and legal. Not having a neutral has nothing to do with it IMO.

I'm not sure where you are going with the idea of installing a 240-480 v transfomer. I'd be interested to hear your reasoning for this idea.:smile:

 

[coulter]

bobl -
If the load doesn't need a neutral, then there is no reason to run on.

If I understood you correctly, the outside lights are fed with a 1ph 408V circuit. And there is a grounding conductor from the 480V panel to each light.

Assuming you have a grounded 480V system, then that grounding conductor should be bonded to the 480V neutral at the panel (or xfm) and also connected to the metal frame at the light. So if you get a grounded hot conductor at the light, the return path is the grounding conductor back to the panel, to the neutral.

The ground rod at each pole doesn't have anything to do with tripping a CB for a ground fault.

Lightning strikes are another issue. Not the same subject as tripping a CB for a ground fault.

carl

 

[bobl]

Thank you for your response:

Q: Where is the TVSS installed?

A: At the lighting distribution panel through a 3 pole breaker.
It appears as though it was installed line to line and grounded. (See attached Photo)
http://forums.mikeholt.com/attachment.php?attachmentid=1594&stc=1&d=1209143812
Q: At the 480 panel feeding the lighting? What modes does it protect? Line to line, line to neutral? What is the initial clamping voltage? Are you sure that the lightning is entering in through the 480 circuit feeding the exterior lights? How do you know?

A: The attached photo occurred during a lighting storm. The cover ended up across the room.

http://forums.mikeholt.com/attachment.php?attachmentid=1591&stc=1&d=1209143812

Q: How is the 120/240 -480/277 V transformer grounded? Is the transformer specially made for stepping up voltage or is it "reverse wired?"

A: The transformer is intended for stepping up. With the case grounded and the neutral ground on the primary. (GE 9T23Q3804G15 - Primary 208/ Secondary 277/480)

Comment: One misconception is that a circuit breaker will protect equipment from a lightning strike or voltage surges.

Response: I am aware of that and am not sure what the breakers do in this situation with 2 ungrounded legs and no neutral return.

The exterior lighting circuit you described with 2 hots and a ground is common and legal. Not having a neutral has nothing to do with it IMO.

I'm not sure where you are going with the idea of installing a 240-480 v transformer. I'd be interested to hear your reasoning for this idea.

Response: To allow the neutral to return faults from the lights shorted ballasts, etc. I don?t see that possible unless there is a path to the breaker, otherwise if the local neutral is connected to the ground rod, as in the case of some of the high mast lighting, there is a need for an additional breaker in the pole for those applications. Using a single phase gives a neutral return.

 

[bobl]

Coulter

It seems as though that may be the issue. I will need to go back to the site and see if the grounding is complete. (Physical ground Testing)

The load does not need a neutral.

The system is (As far as can be seen) a grounded 480V system and the grounding conductor should be bonded to the 480V neutral at the panel and is also connected to the metal frame at the light.

It appears that the situation may have been caused by lightning and the grounding system may not be in tact. So when we get a grounded hot conductor at the light, the return path is the grounding conductor back to the panel, to the neutral is not present.

Sometimes I forget how powerful lightning is and the far reaching the effects could be. Several transformers have been blown out and replaced but the grounding system may not have been tested.

Thanks for the insight.

 

[coulter]

... I will need to go back to the site and see if the grounding is complete. ...[/quote]
You may also want to check the bonding.

bobl]... It appears that the situation may have been caused by lightning and the grounding system may not be in tact. ... ...the grounding system may not have been tested. ...

You may also want to get a copy of NFPA 780. See what they recomend for lightning protection.

If you have lightpoles wirh a concrete/rebar base, I've never understood what the addition of a ground would do to help mitigate damage from lightning.

carl

 

[nakulak]

I'm a little confused - are the lightning strikes occurring on the poles (load side of your panels) or on the grid (line side of your panels somewhere) ?

generally tvss are used to protect your premise wiring from incoming surges on the grid

protecting your premise wiring from lighting stikes within your premise is another matter entirely

do the light poles have fuses at each pole ?

 

[Pierre C Belarge]

I am also curious as to where the lightning strike is occuring. This seems like the strike would be very close to the TVSS/building and not located out by a lighting pole.

A lightning expert is most likely the one who can actually help you, as he can see the actual site and make better judgements.

 

[nakulak]

I was perusing the squareD site, as I was curious about your comment on the single phase protection. It appears that they have products which meet your need exactly (see link), and I think that the suggestion you meet with a lighting protection professional is spot-on.

http://www.squared.com/us/products/surge_protection.nsf/unid/69C8B19A6011A07B85256A17005D5766/$file/surgeprotectionFrameset.htm
(see the SDSA secondary surge arrestors)

 

[mxslick]

A good source of help...

A good source of help...

....would be to contact the manufacturer of the TVSS, send them the photos of the damage you've shown us (they may also want the damaged unit too...) as they can analyze the failure mode(s) of the TVSS and advise a possible solution.

A lot of manufacturers (for example APC for UPS devices) appreciate it when customers share info on such issues so they can make corrections or design improvements.

 

[bobl]

The problem is compounded by the use of the step-up transformer to feed the lights in the building with 277. Any spike/surge that gets back to the transformer can directly affect the lights in the building. A number of ballasts have been blown out by a strike.

The main question here seems to be answered in the responses. The 480 volts feeding the lights from a 277/480 volt transformer is permissible. The use of 2 ungrounded wires feeding the lights is OK as long as there is a grounding wire on the structure and is connected to the neutral at the panel.