Grounding & Protection on streetlighting

[fsck]

In 2004, a PhD student was electrocuted in NYC by a hot junction box cover. I was quite interested in the case because a friend's Seeing Eye dog was similarly killed (in another city..) a day or two later.

Jody Lane's family settled with ConEd by establishing a foundation in her name

http://strayvoltagenyc.org/

to reduce if not eliminate such hazards. They've pushed ConEd into doing surveys for other exposed sources, and found tens of thousands so far. Many of these seem to be streetlight circuits.

I know well how corroded buried jboxes and conduits get in a snow+salt environment, but aren't boxes required to be grounded independently of the conduit? And what fault protection is found on streetlight circuits in general?

 

[waross]

I know of one Canadian city that years ago started bonding the neutral to every standard or post.
I realize that this does not address the issue of junction boxes and their covers but exposed junction boxes were very rare in that system.
The same method woul;d provide protection to metalic boxes and covers.
Unfortunately, in Canada, this option is only available to Cities and utilities who may choose to exempt themselves from the electrical codes.
When many of the old street lighting circuits were installed the code and practice was to rely on individual grounding at each standard. In hot dry weather, there may not always be enough ground conduction to trip a breaker should a hot wire or failed ballast energize the standard. Then a quick downpour may give good surface grounding while the typical grounding plate on the bottom of the concrete base is still in a high impedance ground condition. This may result in a condition where a fault energiizes the standard but there is not enough fault current to cause a breaker trip but a person touching the standard may be well grounded by the wet surface soil. It was a number of cases such as this (but fortunately no fatalities) which led the city in question to undertake to bond the neutrals to each standard.
For a private installation, running a dedicated ground conductor will give good protection, There may still be a slight hazard as a broken ground connection will usually go undetected. A neutral bonded to the standard will trip the circuit breaker in the event of a failure to ground. You may petition the AHJ for an exception to allow bonding the neutral to each standard. Some may allow it, some won't.

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

 

[dpc]

I agree with Bill, the old way was to provide only a ground rod at each light pole and no ground wire. This sounds good, but can create some serious safety issues. A grounding conductor should run to each light pole back to the source to increase the odds that a hot wire touching the pole will create enough fault current to trip the breaker. It's not perfect, but it's a big improvement.



David Castor

http://www.cvoes.com

 

[rasevskii]

Having just now looked at the abovementioned strayvoltage website, they mention 'one volt or more', excuse me, but just how do you measure that with a DVM, where is the ground potential connection for the DVM? Do they just stick an electrode into the earth somewhere to get the reference ground?

In a large city, what with a DC subway system also, there are very possibly hundreds of amps flowing around all over any conductive path resulting in possibly volts of difference between adjacent metal objects in the street, as said, manhole covers, grates, railings, fences, streetlight poles...

What is the criteria, how many volts difference between objects is the limit...We know that in power installations, hundreds of amps can be flowing around in metalwork, if one can get a clamparound meter around something, current is flowing. Therefore voltage drops exist also. Normal.

Say 20 or 30 Volts limit. But that will vary with time of day and the loading on the system. And if a subway train happens to be passing through the area or not.

Sounds like Lawyer's Paradise...

rasevskii

 

[fsck]

I assume they are looking for 60 Hz E-field, which keeps NYCTA out of the picture.

The real question I have is about integrity of the grounds on flush ground level boxes.....

 

[rasevskii]

Just how do you measure the 60hz E-field from a ground level box cover? By some sort of capacitive pickup device waved over the top of it? Rather a good technology has to have been developed for this, with filters to eliminate unwanted frequencies. From a moving vehicle ? Interesting.

BTW the DC supplies for subways and other rail is from silicon rectifiers that produce their own sets of harmonics, as well as the modern traction control systems on the trains that use IGBTs and other methods involving HF impulses.

It is very noisy out there.

rasevskii

 

[fsck]

{noisy out there} Oh, that it is!

I wonder what the harmonics looked like in the Good Old Daze when NYCTA used synchronous converters. They were bidirectional.

(Amazingly, Amtrak not only still has some rotaries in service and just spent $63E6 rebuilding the 1920 era "Lamokin Converters" in Chester PA; but they are 60hz->25, not 25->DC)

As for how they survey, see

http://www.powersurveyco.com/technology.html

 

[waross]

It's pretty simple to measure. Use a high impedance volt-meter. Stand beside the box in question. Grasp one lead in your hand and touch the object in question with the other lead. Then push the first lead an inch or so into the earth. If there are any grounded objects nearby check the voltage from them to the jubction box.
This is a real problem with street lighting circuyits that were wired to previous electrical codes.
This is not stray EMR. This is not high magnitude currents through the earth from subways or substations.
The problem is some type of insulation failure to the metal of a junction box or lighting standard under conditions where the grounding impedance is too high to trip a circuit breaker. The breakers on a legacy street lighting circuits are often 15A or 20A.


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

 

[resqcapt19]

Bill,
You said "This is a real problem with street lighting circuyits that were wired to previous electrical codes." Most of the street lighting is installed bu the utilities per the National Electrical Safety Code (NESC) and not to the National Electrical Code. Has the NESC been changed to require the use of an equipment grounding conductor?

 

[fsck]

It's pretty simple to measure....
.....
The problem is some type of insulation failure to the metal of a junction box or lighting standard under conditions where the grounding impedance is too high to trip a circuit breaker.

What I was pondering was a test of just that; a source against a test ground that looks for the box's ability to actually sink current at the spec'ed elevation above the ground. But I suspect ConEd would find literally 100's of thousands of floaters..

 

[waross]

My streetlighting days are long gone. I don't know the present practice but street light systems have a very long life.
I knew an old electrician who was close to retirement. He had served his apprenticeship installing series street lighting circuits. When he retired he was still servicing the circuits that he had installed as a young man.
And, I was in Canada. Our city generally followed the Canadian Electrical Code, The Canadian equivalent to the NEC. However compliance was voluntary and for special cases such as street lighting we deviated from the code.
Given the service life of street lighting circuits I though my contribution may be valuable.

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

 

[rasevskii]

for fsck.

Well, indeed, an interesting technology from Powersurveyco.
If one ever sees one of these coming down the street, then we will know what it is...

Here in Europe we have plenty of possible uses for this. There are 110kv cables under sidewalks, and even 400KV cables in ducts under walkways in certain locations. Unlike the USA, cables are direct burial, or in concrete or tile fittings 2 meters (about) below the street surface. Not in pullable duct banks or conduits with manholes at intervals.

Yes on the old synchronous converters (rotaries), today no longer existing anywhere, I happen to know a lot about these machines. BTW I was involved in the first 60 to 25hz static frequency converter installed in the USA in Philadelphia in around 1985.

rasevskii