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Isolation Transformer - 2 wire street lighting

Isolation Transformer - 2 wire street lighting

Isolation Transformer - 2 wire street lighting

(OP)
Old existing 2 wire (120V) street lighting system does not have a 3rd wire brought to each pole for purpose of grounding and presents an unsafe condition if hot leg of phase conductor shorts to a metal pole as a high impedance path (earth) back to the 120V source does not create a good path to trip an OCPD so the pole can remain energized at 100V or so and go undetected.  New installations include a 12/3 and the ground wire connected to the pole metal casing and grounded at each pole with a ground rod.  (gnd not bonded to white conductor for new installs)

As a possible solution, has anyone used an isolation transformer (120:120V) fed from a pad-mount utility transformer connected so as to provide a safer situation.  The isolation transformer line side is connected to hot leg (120 volts) and neutral in the transformer and the load side is connected to the hot and neutral running to the light(s). Since there are no grounds in the load side of the circuit (light is not grounded) and the load side of the isolation transformer is physically isolated from the grounded line side at the transformer touching an energized light pole causes no current to flow to ground and no circuit to be completed to ground if the pole becomes energized and someone touches it.

Other ideas include installing a pole ground and ground wire at each pole and bond the neutral to the ground at each pole, thereby creating a grounded neutral path back to the source to trip an OCPD. (similar to a multi-grounded service that a utility provides to houses in that a grounded neutral is provided from the utility transformer to the house where the service is then grounded per NEC)

Thoughts/comments or code related descriptions regarding reasons not to use an isolation transformer would be beneficial.  As far as code, NESC applies here and not the NEC as this is a utility owned equipment.
 

RE: Isolation Transformer - 2 wire street lighting

It's been many years since I did street lighting troubleshooting but as I remember, We had hot to pole faults as often with ballasts as with incandescent lamps. Your isolation transformer would probably introduce as many issues as it solved. Decades ago our municipal standard was changed to require bonding the neutral to the pole as well as a pole ground at each pole. As a city we were not subject to the National code.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Isolation Transformer - 2 wire street lighting

The bonding and grounding of the neutral at each pole will solve the problem of a pole becoming energized by a fault, but the pole and any downstream poles would become energized by an open neutral on the supply side of the pole.

RE: Isolation Transformer - 2 wire street lighting

(OP)
Thanks resqcapt19.

What's your take (or others) on using an isolation transformer as described in the 1st post?

What issues do you have regarding the isolation transformer?



 

RE: Isolation Transformer - 2 wire street lighting

There may be enough capacitive coupling on the load side of your isolation transformer to still create a shock hazard in the event of a ground fault on the load side.  

What about using GFCI protection on the circuit that feeds the lights?  If installed at the power source, the GFCI would open the circuit on any ground fault.  Note that if you have long runs to the lights, the total leakage current might exceed the GFCI trip point making this solution unworkable.

RE: Isolation Transformer - 2 wire street lighting

(OP)
I agree with your statement that an isolation transformer may create a shock hazard in the event of a ground fault on the load side.

The 120V circuit could feed up to 3 or so lights to where a GFCI may nuisance trip due to excess leakage current - I agree that this may not be a workable solution.

I'm trying to build a case for reasons to justify not using an isolation transformer as a viable solution:

1. Lighting surge may create an overvoltage to the isolation transformer secondary damaging the equipment, NEC does not allow an isolation transformer for lighting circuits (only allowed for hospital receptacle ckts found in critical areas, recording audio studio equipment, and motor control circutis), and I'd question the isolation transformer expected service life.

On a separate issue, I need to come up with a workable solution to make it a safer installation than what presently exists and at the same time comply with NESC.

With that being said, if anyone has suggestions/comments to either of the above last 2 paragraphs, I'm all ears.  I don't believe there is a good solution yet, other than to add the ground wire and connect the ground wire to the metal poles, which is a very costly solution.

Any solutions or other reasons to not install isolation transformers are welcomed.

Thanks!





 

RE: Isolation Transformer - 2 wire street lighting

I agree that the best solution would be the addition of an equipment grounding conductor.  

Until the 2008 NEC, it was permitted to run a feeder to a second building or structure and not run an EGC with the feeder.  The grounded conductor would be re-bonded to a grounding electrode at the second building or structure.  This was changed in the 2008 code to require the use of an EGC with all circuits that are run to a second building.

The addition of a grounding electrode and the re-bonding of the grounded conductor at each light would be what was permitted in the 2005 and earlier editions of the NEC and is probably the third best solution to the issue at hand. The first being the installation of an EGC and the second being the protection of the circuits with a GFCI type device.  While not permitted as "people" protection by the NEC, you might consider the use of an equipment protection ground fault device with a trip of 30 mA.  A ground fault to earth would likely flow enough current to open the circuit, but not trip on the normal leakage current of the circuit. (a liability issue for the engineer as a 30 mA ground fault trip device is not intended to protect people from a shock hazard)

Given that this is an existing circuit, you might argue that the exception to 250.32(B) in the 2008 NEC would permit the re-bonding of the grounded conductor to a grounding electrode at each pole.  However, the addition of a grounding electrode and the re-bonding of the grounded conductor at each light still presents a shock hazard in the event of a open neutral on the line side of the light.  This could be minimized with a ground ring around the pole so that a person standing on the ground and touching the pole would be exposed to a limited voltage, but the ground ring would introduce a step potential hazard.  (again, this would probably create a liability issue for the engineer who designs this and approves its installation)

I can't comment on the NESC as I am an electrician and do not work with that document.  

 

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