Mike Holt's - Grounding safety fundamentals video question

[Xiggy]

So I just got done watching this more than an hour long video. As far as I understood, in a building with lots of equipment, lightning can induce voltages into insulated conductors and to prevent cross arcing you ground the metal equipment to the ground to so it has a place to dissipate to. But then later in the video it's talking about how you should not just install ground rods everywhere (auxiliary electrodes) so it doesn't try and travel through equipment and dissipate through a separate equipment ground.

At first I thought ok, then just install grounded electrode conductors to a grounding electrode in a ungrounded system, because those don't have a place to dissipate the induced voltage, however on a second video of his he says grounded AND ungrounded systems. Then later on on that second video I understood it as bond everything metal, and give it just one solid grounding electrode. Which is it?

As a separate remark, he focuses on induced voltage in transformer windings due to a magnetic pulse from lightning - but wouldn't the transformer enclosure act as a faraday cage, and in essence no magnetic field should get inside to produce that induction? I get it in cables outside, because they're unsheathed, or maybe in plastic conduit. Anyway, just an observation.

 

[George Stolz]

I'm assuming you're referencing the video at 46 minutes:

I'm really not drawing the connection you're making in your statement. It seems like a pretty clear explanation in the video? You can't generally flip to an ungrounded system on a whim, there's a lot of requirements for doing that and you wouldn't be able to modify the equipment to function on it.

On the last remark, lightning hits transmission lines and passes through transformers regularly.

 

[Joe.B]

There are two things you need to to make sure you have clearly distinguished in your mind, I know I did.

1. The difference between "grounding" and "bonding".
2. The difference between "conductive" and "inductive".

I think that if you can confidently define those terms, you'll no longer be confused.

 

[jaggedben]

To oversimplify a complicated issue...(and show my age)...

When a lightning strike hits the ground, in injects a huge amount of charge that needs to dissipate. It can be thought of as instantly creating a voltage gradient between the point of the strike and everything around it. That is, if you were to measure voltage during those moments between any point closer to the strike and farther away from the strike, you'll have at some instant a positive voltage wanting to push the charge (current) away from the lightning strike. More conductive material, i.e. metal, will conduct more of that charge. The voltage involved with lighting is initially also high enough that it can jump through the air where it meets resistance (as it already did through the sky).

So the wave of charge dissipation from a strike is coming along through the ground and it meets a building. Any grounding electrodes and underground metal raceways or conductors that the wave directly meets will definitely conduct more charge than the general surroundings. If all these electrodes are connected to each other at the ground, say to a single grounding electrode conductor, with 8awg or larger bonding jumpers, then the charge will be more likely to dissipate through the 8awg+ wires back to the ground. However say an old incoming copper phone line is grounded to its own rod at one side of the building and the electrical service is grounded at the other side. When the lighting strike charge wave arrives, there is a voltage between the grounding electrodes on opposite sides of the building and the charge is looking for a conductive path between them. Now the lighting charge could travel up from the phone line ground rod to the phone NID, jump (flash over) the short distance to the phone line wiring, travel along the 22awg phone line wiring to a phone jack, where it flashes over to the power wiring for the answering machine, travels over that wiring to the electrical service and back to ground via the service grounding electrode conductor. Mostly a great conductive path but way too small for the current. So the phone line wiring, answering machine, and possibly the 14awg power wiring for the answering machine outlet are all damaged or destroyed.

So what Mike and others would advocate is that you install your building grounding as required by the NEC, with the generally beefier required conductors, and then anything else that might need grounding you connect together above ground, so as to reduce the chances that lighting jumps out of the ground in one spot and damages interior stuff finding a path to where it can go back to ground to further dissipate. And they advise not to drive extra ground rods for things that might create these parallel paths with the earth over small wiring. It's why it's now required to install an Intersystem Bonding Bridge so that communications requiring grounding can connect to the electrical service grounding.