Isolation Transformer for Energized Metal Pole?

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tom baker

tom baker

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I received the following email and am passing it along for comments:
We have a somewhat unique situation in that an isolation transformer mfg is proposing to add a 120:120V isolation transformer within a pad-mount utility transformer as a solution to re-feed existing street lights that are presently fed by 120 volt, 2 wire circuits without a ground > conductor brought to each metal street light circuit.

The isolation transformer mfg has indicated it will make a safer installation as a
solution to solve the existing situation of: an existing metal pole was energized at 90-100 volts (measured from metal pole to earth) as an internal fault to the pole occurred and did not clear an upstream over-current protective device and the pole remained energized.

As a temporary solution, the utility installed a ground rod and bonded the rod to the light pole to no avail, which resulted in the same measured touch potential (90-100V) as before and the measured current increased from 4 amps to 10 amps, since no ground conductor existed to create a low-impedance path back to the source to trip an OCPD. To come up with a low cost solution, the utility found an isolation transformer manufacturer that has applied their iso xfmr out east as a solution in > lieu of adding a ground conductor. (approximate cost = $1M to add ground conductor)
 
I have done shielded Isolation transformers at marinas with shielding bonded only to the primary side as the only way to stop voltage drop on the MGN from destroying the hulls of metal boats, but the secondary's still had to be grounded, if the current is below 60 amps, why not use some sort of GFCI protection instead of Isolation? that way if any ground fault it would open the circuit, either way is an NEC violation of the NEC, if you can go under the NTSC you might have be able to do it, as they allow the neutral to be used for fault current path.
 
tom baker said:
The isolation transformer mfg has indicated it will make a safer installation as a
solution to solve the existing situation of: an existing metal pole was energized at 90-100 volts (measured from metal pole to earth) as an internal fault to the pole occurred and did not clear an upstream over-current protective device and the pole remained energized.
Click to expand...
Silly question ,... How will the isolation transformer remedy this?
 
hurk27 said:
if the secondary is not bonded to earth there would be no referance voltage to earth.
Click to expand...

Unless the distance is enough that capacitance coupling still provided enough current to kill ya.

Regardless it would not be acceptable under the NEC, I wonder if it is acceptable under the NESC?
 
found this thought I'd share it. http://grouper.ieee.org/groups/td/dist/stray/files/2006-11-DeNardo.pdf

they are apparently using isolation transformers In NY . Interesting,. so the pole in Tom's example is still energized but because the of the fact that the xfmr secondary neutral is not earthed there is no path.. so I guess no objectionable current.. hmmmm not sure what to think. except that an equipment ground would be a much better solution .
 
When I was in the Navy, our shipboard electrical systems were ungrounded, but we were cautioned that we could still get shocked due to capacitive coupling.

The isolation transformer may reduce shocks by 70%, but what about the other 30%?
One death is all it would take.
My comment to the person who sent me the email was the POCOs don't want to be under the NEC, which is prescriptive, but prefer the NESC which is performance based, and thats why we have issues such as this. I feel the POCOs should follow the rules of the NEC for grounding and bonding on systems 600 V and under, particularly for street lighting.
 
Tom would the fact that the isolation transformer is in the base of each pole mitigate the capacitive coupling effect ??
 
I'd be concerned that using a gfci breaker may have enough leakage current to cause a nuisance trip when the gfci breaker feeds more than 1 light.

What is IMHO?

Equipment ground is best solution here, but costly.

Iso xfmr may create a shock hazard due to capacitive coupling and NESC would not allow Iso xfmr, correct?
 
ydsprki said:
I'd be concerned that using a gfci breaker may have enough leakage current to cause a nuisance trip when the gfci breaker feeds more than 1 light.
Click to expand...
If there is such leakage current, then you have a real problem, a real danger, on your hands. If the GFCI trips, then it is correctly doing its job.
ydsprki said:
What is IMHO?
Click to expand...
"In My Humble Opinion."
 
My opinion is that this is a 'band-aid' fix.

There is a fault between a circuit conductor and the pole that has to be repaired.

Adding an isolation transformer without fixing the fault simply makes this into an 'unintentionally grounded system', where the potential of the faulted conductor is reduced to that of ground at the fault.

Under earlier code, a circuit to a detached structure could be run using the grounded conductor as the bonding conductor for fault current. This might apply to the installation being discussed, as a repair.

-Jon
 
I was there.

I was there.

Guys, you can isolate Elevated Neutral to Earth Voltage (NEV - Stray Voltage) with a transformer if the transformer case is nonmetallic; so this would work for an individual circuit. Also if you are a utility and you follow your own rules, you don't have to ground the secondary, so electric shock hazard is reduced, again for an individual circuit.
I was on the committee at Jody Lane Foundatin with ConEd when this product was introduced.

But for premises wiring, we have service equipment (disconnect) that must be grounded, and the secondary must be grounded, so it doesn't work for us, or for the utility if it's more than a single circuit.
 
Mike,
Wouldn't there be a metallic frame inside the transformer enclosure that would have to be bonded to both the primary and secondary EGCs? (assuming NEC rules)
 
Nesc

Nesc

Hello Mike,

If the utility complies with NESC 092.B.1, then what exception in the NESC can a utility go by for lighting circuits as it states for 750V and below for a 1-phase system with an associated lighting circuit, the point of grounding connection shall be on the common circuit conductor associated with the lighting circuits.

Point is, if the isolation transformer is installed at each pole, the source side of the isolation transformer (120V 2-wire) must have a grounded neutral as a faulty wire inside the pole could energize the pole. So if your going thru the effort of bonding and grounding the source side of the isolation transformer neutral circuit conductor, then why spend the extra $ on the isolation transformer?

What NESC code allows the utility to have an ungrounded conductor at the street light?

Thanks for your input!
 
This electrical configuration is similar to hospital operating rooms.They however install very sensitive ground current leakage detectors that will completely trip this electrical system.
 
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