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Lighning Protection in Metallic Enclosure

Lighning Protection in Metallic Enclosure

Lighning Protection in Metallic Enclosure

(OP)
Hello all,

Considering the Faraday cage shielding principle, here are two questions associated with lightning protection:

1)Is a metallic enclosure effectively grounded self-protected again lightning strike?
2)Can this principle could be applied to an outdoor metal clad switchgear in distribution substation application?

RE: Lighning Protection in Metallic Enclosure

I think you have to determine what you are trying to protect against. The original purpose of lightning protection systems was fire prevention. Based on this, metal buildings and enclosures are generally adequately protected. We don't need lightning protection systems for metal buildings, including metal-clad switchgear. But you would still have to consider surge protection for the electrical system components. That's a different issue.

RE: Lighning Protection in Metallic Enclosure

(OP)
Thanks DPC.

There any difference in surge protection for a switchgear with or without a lightning mast around the switchgear?

If there is a microprocessor relay and underground feeders in a switchgear, there is any recommendation to protect the switchgear in the event of direct lightning strike?

Thanks in advance

RE: Lighning Protection in Metallic Enclosure


1- No. lightning mast will protect switchgear against a lightning strike.

2- No. Surge protection is used for potential transformer, current transformer , any power equipment with winding.

RE: Lighning Protection in Metallic Enclosure

I disagree with odlanor regarding the need for a lightning mast on a metal building.

Your electronics should be provided with appropriate surge protection.

But, if the building experiences a direct strike, air terminal or no air terminal, there are no guarantees. You're dealing with a huge charge that just traveled maybe 20,000 feet through the air. It's difficult to totally control how it dissipates.

RE: Lighning Protection in Metallic Enclosure

dpc,
you're right! This is only an additional shielding proposed by cuky2000.

I have seen 3 direct stroke at HV and EHV substation crossing shielding protection. All resulted in damage for CT or PT equipment.

RE: Lighning Protection in Metallic Enclosure

Both dpc and odlanor are not wrong. By providing lightning mast and dedicated downleads will give a low impedance path to dissipate the current to ground as oppose to let it find its way and sparkles between metallic building members.

RE: Lighning Protection in Metallic Enclosure

(OP)
Hi Pwrtran and members,

Here are a few more questions to motivate the dialog in this subjet:

1) There is any indication that the impedance of lighting mast vs. the impedance of a metal clad switchgear effectively grounded will provide significant difference protected the equipment inside?

2) Any concern regarding back-flashover on the gear after a direct strike in a nearby lightning mast?

3) Let's assume for a minute that the lightning mast and the metal clad switchgear have equal impedances to ground. How will be the overvoltage stress imposed on equipment such as relay, PT, CT breakers, cable control wire etc. could be dissipated in the event of a direct or nearby lightning strike?

4) Does any one could help with reference or advise were to find statistics regarding lightning damages in metal building or shielded enclosures housing electrical equipment.?

Your help will be appreciated

RE: Lighning Protection in Metallic Enclosure

I was under the impression that the purpose of a lightning rod was to avoid strikes, not conduct them. The rods are sharply pointed to encourage the greatest corona to discharge the field before it can build up to flashover potential.

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

RE: Lighning Protection in Metallic Enclosure

waross,
the purpose of a lightning rod is to attract strikes, and conduct them to ground.

RE: Lighning Protection in Metallic Enclosure

There are two parts that contribute to the potential raise during a lightning strike - the resistance portion and the inductance portion.

The resistance is determined by the ground grid resistance plus the resistance of the down leads. Consider all equipment is on a common ground then the potential raise due to the dissipation current flowing through the ground grid is same. However, the direct lightning strike current is an impulse like that has a typical 1.6µs front and 50µs tail. Therefore, the potential of the inductive portion is L*di/dt per linear length of the down leads, which could be the dominate part. So, even other equipment is on the same ground they only share about the resistive portion but not the L*di/dt part, so back flash over could happen.

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