Low Voltage Grounding for Lightning

[dolsonmch]

I'm a consultant for Audio-Visual Systems. Fairly regularly we design systems that have an outdoor component to a mostly indoor system. Our concern is grounding for low voltage copper lines that go from outside plates or equipment locations into an equipment rack room inside. Is there code that we need to follow for this scenario? I've looked at the stuff that TPD makes and that can work, I'm just wondering if it is required.

 

[tom baker]

What is the voltage?
Are you using an Art 411 system?
Are the lights fed from a GFCI?
What is an outside plate?
Who or what is TPD?

 

[dolsonmch]

Low voltage. AV system, not lighting. We're concerned about code requirements for a Lightning strike happening nearby and what we need to do to mitigate that voltage to keep it from going into the building.

 

[tom baker]

Ok sorry I saw AV and thought lighting
What is the voltage?
What is a plate?
You’ll want SPD on all lines/wires in and out of building and a single point ground
If lightning hits your outside equipment it’s gone
You may be able to prevent damage to inside gear.
And SPDS on inside gear

 

[dolsonmch]

can you tell me where those rules are in the NEC? I've looked through a number of books and haven't been able to find specifics on LV wiring related to lightning

 

[tom baker]

What voltage please?
To a poco line worker 120 v is low voltage.
What is a plate?

 

[dolsonmch]

Voltage. It's audio visual equipment, so it varies. If it's a speaker line it could over 120v.

Plate. Connection plate in a box with multiple connections.

Here's an example of a plate:

 

[zbang]

You probably won't find what you're looking for in NFPA 70 (the NEC) although there might be something in article 800. Also, BICSI may have something relevant. (I'm too lazy to check them at the moment .)

Past that, you're more into design principles and away from fire prevention, which is the purpose of the NEC. Speaker-level lines should have SPDs (surge protective devices), line-level should have them, too, but they could be lower voltage devices. If you can, make sure that anything not speaker-level is transformer-isolated, which will help with both lightning-induced surges and ground loops.

 

[dolsonmch]

You probably won't find what you're looking for in NFPA 70 (the NEC) although there might be something in article 800. Also, BICSI may have something relevant. (I'm too lazy to check them at the moment .)

Past that, you're more into design principles and away from fire prevention, which is the purpose of the NEC. Speaker-level lines should have SPDs (surge protective devices), line-level should have them, too, but they could be lower voltage devices. If you can, make sure that anything not speaker-level is transformer-isolated, which will help with both lightning-induced surges and ground loops.

so just for clarification, you're saying there really isn't any code in regards to this, it's just "best practices" and grounding as much as possible any copper wiring that goes from outside to inside, right?

 

[petersonra]

There is nothing in the NEC that will protect your circuits from lightning. That is a design consideration. The grounding and bonding requirements found in the NEC are there for protection of human beings, nit equipment, although they may have a secondary effect of protecting equipment as well as human beings.

I think most audio circuits would be covered in article 725.

 

[tom baker]

Article 725 can be overwhelming to understand. There is an NEC article on SPDS, it’s 285, but it is for line voltage
You will be better off finding a forum for AV equipment for answers.
But to start you will want SPDS on all wires in and out.

 

[tom baker]

You want a single point ground. All the audio line SPDs are bonded on a common bar, then that is bonded to the buildings GES. What your are doing is very similar to how cell towers are done. Search for a copy of Motorolas R56,

 

[hbiss]

I know I've used SPDs for 70.7V outdoor speaker lines so no problem there. Never really had to protect any line level or mic level audio lines.

This is the current generation from Ditek that I use for 70V lines:

https://www.diteksurgeprotection.com/component/jdownloads/?task=download.send&id=160&catid=17&m=0&Itemid=101

Note that those modules are available with clamping voltages from 6.8 to 108 volts. The 6.8V would be good for low level audio. If you have speaker lines over 100V I would give them a call for a recommendation.

EDIT: I disagree that the NEC requires SPDs on this type of wiring. Not withstanding that it is prudent to provide it.

-Hal

 

[tortuga]

so just for clarification, you're saying there really isn't any code in regards to this, it's just "best practices" and grounding as much as possible any copper wiring that goes from outside to inside, right?

I think most audio circuits would be covered in article 725.

The NEC has specific rules for audio systems in Article 640, the rules on grounding are in 640.7
The only thing about audio circuit in article 725 is that they cannot share a raceway with regular (non audio) class 2 or 3 cables 725.139(F)

 

[tortuga]

I should add parts of Article 640 pull in article 725 and thus 725.141:

725.141 Installation of Circuit Conductors Extending Beyond One Building. Where Class 2 or Class 3 circuit conductors extend beyond one building and are run so as to be subject to accidental contact with electric light or power conductors operating over 300 volts to ground, or are exposed to lightning on interbuilding circuits on the same premises, the requirements of the following shall also apply: (1) Sections 800.44, 800.50, 800.53, 800.93, 800.100, 800.170(A), and 800.170(B) for other than coaxial conductors (2) Sections 820.44, 820.93, and 820.100 for coaxial conductors

 

[hbiss]

The only thing about audio circuit in article 725 is that they cannot share a raceway with regular (non audio) class 2 or 3 cables 725.139(F)

725.139(F) The only audio wiring that cannot share a conduit or raceway with other CL2 or CL3 wiring is amplifier audio output wiring for speakers. See 640.9(C). That would be your 70 and 100 volt lines.

Art. 800 pertains to communications circuits that originate from a "communications utility" that enter a building. Examples include telephone and cable TV/internet. So, I don't think your application of wiring to outside jack panels applies.

800.90 talks about protective devices:

800.90 Protective Devices.
(A) Application. A listed primary protector shall be provided
on each circuit run partly or entirely in aerial wire or aerial
cable not confined within a block. Also, a listed primary protector
shall be provided on each circuit, aerial or underground,
located within the block containing the building served so as to
be exposed to accidental contact with electric light or power
conductors operating at over 300 volts to ground. In addition,
where there exists a lightning exposure, each interbuilding
circuit on a premises shall be protected by a listed primary
protector at each end of the interbuilding circuit. Installation
of primary protectors shall also comply with 110.3(B).

Note the use of the word "block" which the NEC defines as:

Block. A square or portion of a city, town, or village enclosed
by streets and including the alleys so enclosed, but not any
street.

So, we don't know how far or how you intend to run your wiring. Art 800 won't specifically apply but there is some good advice given. Note that the NEC is a minimum requirement and not a design guide. So, if it were me, I would want to protect my equipment as best I know how.


-Hal

 

[tortuga]

725.139(F) The only audio wiring that cannot share a conduit or raceway with other CL2 or CL3 wiring is amplifier audio output wiring for speakers. See 640.9(C). That would be your 70 and 100 volt lines.

Good clarification, yeah a huge mistake happened around that one in the 90's so I always remember it.

Art. 800 pertains to communications circuits that originate from a "communications utility" that enter a building. Examples include telephone and cable TV/internet. So, I don't think your application of wiring to outside jack panels applies.
800.90 talks about protective devices:

Yeah 800 does not apply except for where 725.141 pulls in specific sections of article 800 like I quoted in post 15