Preferred connection for lightning arresters

What is the preferred location for lightning arresters installed on underground cable taps off overhead lines?
For years, the standard circuit was: overhead conductor tap -> fused (or
solid) cutout -> lightning arrester -> underground cable. That's the way I've always spec'd them.
I guess the idea was that the arrester would continue to protect the cable from lightning even if the cutout was opened. In addition, the cutout fuse would protect the overhead feeder in the event of an arrester failing shorted.
Recently, I observed our local utility (their contractors, actually) connecting these in the following sequence: overhead conductor tap -> lightning arrester -> fused cutout -> underground cable.
Here, if the cutout is opened, the cable is no longer protected by the arrester. In addition, a shorted arrester must be cleared from the feeder by the station breaker.
We don't get much lightning in the Pacific Northwet, so this isn't as big an issue as in the midwest or southeast. But then, our utility people don't get as much experience in getting this stuff right either. What's everyone's opinion on either way of hooking these up?
--
Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
note to spammers: a Washington State resident
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Quite unlikely!
Around here, "they" put in a set of lightning arresters for every 3 poles regardless of whether there is a tap. It could be that your utility figures it pointless to put an arrester on the cable just a foot away from an arrestor protecting the main feeder.

The cable may not be "protected" against lighting at that pole, but a close strike might well put the cable at risk even with the arrester. If lightning hits "down the line" the open fuse will isolate it.
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John Gilmer wrote:

By 'here' I take it that you mean an area that experiences a lot of lightning. Western Washington state isn't one of those and as a consequence, lightning protection is only used to protect underground cable risers and high value cpital equipment (substations, etc.).

There are cases in which the cable riser might be the alternate feed to an underground system. If the strike occurs near the primary u.g. system feed, the voltage at the end of a line due to an impulse can double at an open circuit due to reflection. Leaving an arrester connected at such an open can reduce its magnitude.
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I would not care to quantify it, but we (Northern Neck of Virginia) have a "fair" amount of lightning.
Because of all the cell phone and water towers here abouts, I suspect that direct lightning strikes on the 19.8 KV (phase to ground) are rather rare.
The present rules call for ALL secondary drops to be underground. In areas with lots smaller than an acre or so, "new" single phase HV (19.8) is underground. Because of installations and other incidents, the local utility workers gain a lot of experience and don't have much of a problem repairing or replacing underground wiring. The high voltage stuff is done "cold" (every pad mounted transformer in a "string" permits the downstream transformers to be "unpluged") and there is no danger of falling from a bucket!

close
Yes, but ... often "unused" underground cables "in reserve" are kept energized. That way, the workers can tell by just checking the fuse whether the line is still good or not. Thus, the "reserve" lines are protected by the pole top arrestors.
Maybe today's cables can be placed in the ground for a few years without being energized and work just fine after a few years but that wasn't always the case.
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For shielded-cable protection, this view is generally supported by IEEE C62.22.1.
A recent article refined this by recommending that the AREA ("triangle" of sorts) bound by the arrestor body and the cable-shield lead and center-conductor connection should be kept to a minimum for most effective protection.
--s falke
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s falke wrote:

Makes sense. The larger the loop, the greater its inductance. Since lightning is modeled by a di/dt wavefront, the greater the inductance, the higher the voltage.
Assuming that there is no advantage to seperating the arrester from the cable with the cutout, keeping them adjacent would assist in minimizing this loop area.
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All the utillities I have worked for put the arrestor connection before the cutout. possibly to save the cutout from opening if the line gets hit and only blows the arrestor. the voltages were under 8000 volts phase to phase. Other utillities that had higher voltages, eg 27.6 kv put the arrestors below the cutout because the arrestors sometimes failed violently (explode) when they were energized with the conductor tap and the lineman was closer to the arrestor while installing the hot tap with a grip all stick. They then started to put the arrestor under the switch so the lineman is further away from the arrestor before it becomes energized while closing the switch with 10 or 15 feet of insulated stick. with the new poly arrestors, its not as much of a problem, but the old porcelain ones were suspect.

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