Isolation Transformer - 2 Wire Street Light System

[ydsprki]

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Existing street lights with metal poles are fed by a 120V, 2 wire source with no ground wire brought to light pole(s) creating an usafe situation to where the pole can be energized with 90-100 volts since a low impedance path doesn't exist to trip and OCPD.

Utility owns lights, so NESC applies. A possible solution was presented to install an isolation transformer (120-120V) within the pad-mount utility transformer to re-feed the existing lights utilizing the 2-wires to each pole, primary of isolation transformer would be fused. If a fault occurs to energize the pole in this case, no ground path would exist due to the ungrounded secondary of the isolation transformer, thereby increasing the safety to the public for the 1st fault.

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, 2.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 3. I'd question the isolation transformer expected service life.

On a separate issue, I need to develop 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'd appreciate any comments.

Any viable solutions or other reasons to not install isolation transformers would be beneficial.

All new light pole standards include running 3 conductors that includes a separate ground to each pole and the ground conductor bonded to the pole. In lieu of retrofitting the older 2-wire installation to bring up to the new standards, do other options exist and still meet code. I'm not sold on the isolation transformer as a solution at this point, but would like to cite code reasons not to go with the isolation transformer idea.

Thanks.

 

[don_resqcapt19]

The following is the last comment I posted to this question on another forum. I am re-posting it here to get the discussion started.

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.

 

[dbuckley]

Seems to me the obvious solution is ground fault detection, ie the humble GFCI.

However: I've never seen public streetlights on a three wire connection. Now granted, I've not seen every streetlight on the planet, but I've seen a few, fed both overhead and underground, and with the exception of those installed in accordance with some ruleset like the NEC (almost always on private ground), they are all two wire. The neutral is bonded to the pole structure to create the ground reference.

 

[tom baker]

All our roadway lighting is 3 wire with EGC. All of Washington State DOT is. Our POCO uses EGC.