Properly installing a grounded subpanel for lightning protection

Question

I'm installing a large antenna on the side of my house opposite the electrical service entrance. The antenna must be grounded, by nature of its design. For lightning protection, it's important that the radios connected to the antenna have a single point ground.

Where the antenna feedline enters the house, it does so though a copper bulkhead, attached to a system of three ground rods by 2.5" copper strap.

If I naively plug the radio into the wall, I have this situation:

The problem here is with a nearby strike there will be a large voltage gradient between the two grounds. That voltage appears across the radio, and things go boom.

In professional installations the building is designed so the electrical service and the antenna feedline enter at the same point. Unfortunately that's not really an option for me: the electrical service enters at the opposite side of the house.

What I'd like to do is install an outdoor subpanel at that single point ground such that every wire going to the radio is grounded at just one point. This way there can exist no voltage across the radio.

I've ordered a QO2175SB Surgebreaker surge protective device along with an outdoor box for it to go in. The radio equipment isn't anything huge, and so doesn't require an additional circuit besides the wall outlet that's already there.

I'm a bit unclear how to install the subpanel in a way which both protects the radio and is safe and code compliant. I've read ground and neutral should not be connected at a subpanel. However, if neutral is not connected to ground at the subpanel, what's limiting the surge voltage on neutral relative to ground? Sure there's the wire bonding the two grounds, but with a fast current pulse like lightning the inductance of that connection renders it ineffective.

Answer 1

This is why surge protectors have N-G protection

Decent surge protectors not only protect against surges on the hot wire(s) relative to the neutral wire, they also protect against surges on hot and neutral relative to the equipment grounding and bonding system (in other words, ground). The surge voltage between neutral and ground will get clamped by the MOV(s) between neutral and ground inside the surge protector to around a few hundred volts, which won't hurt much provided the radio's power supply is properly designed.

However, yours does not appear to do that -- instead it appears that it is designed solely for a main panel application where neutral and ground are one and the same. In your case, you should look for a surge protective device (Eaton has the CH2SPTULTRA which will do the job) which has separate neutral and ground connections.

Needless to say, you should also install a surge protective device on the antenna feedline itself.

Answer 2

This gets rather complicated rather fast, because the requirements of the electrical code partially contradict the practical requirements of radio. Worse, finding someone able to do it properly (or approve it) requires someone experienced in both fields at once - such people are few and far between. Fortunately there's a way to do an end-run around the entire problem.

Use a separately derived service

Since you're already bought into installing a subpanel, you are most of the way there.

Install a supply transformer which has the side-effect of fully isolating the supply from your panel. This causes the new panel to become a main panel rather than a sub-panel. You will need to upgrade the antenna's grounding system to fully satisfy the Electrical code for mains power installations. Since it is a main panel, neutral must be bonded to ground here. You have accomplished what you want.

This arrangement greatly simplifies things. Electrical people will understand what this is - separately derived service is unusual, but not that unusual - and a bit simpler than the complex isolated-ground rules. A future electrician working on this system is not likely to botch it up.

The transformer

This is isolated from your house's regular power system. This is accomplished by using a transformer whose primary is isolated from its secondary, which is most ordinary transformers.

Common sizes of transformer are 1.5 KVA and 5 KVA (a VA is similar to a watt, so 1.5KW or 5 KW - 12.5 amps or 41 amps at 120V.)

Most readily available transformers are configurable for 240V or 480V on the primary, and 120V, 240V or 120/240V on the secondary. You would supply it with 240V from your main panel.

You can also obtain isolation transformers of various sizes, which may be 120V on each side.

The main issue is whether the voltage surge of the lightning strike break down the insulation in the isolation transformer. In this case, there could be unpredictable results. There's no requirement to connect the grounding systems to each other, but a heavy ground connection would help to reduce this voltage difference. Realistically you cannot put enough copper ground wire down to carry the ampacity of a lightning strike.

Answer 3

This is why grounding can get complicated. Let's start with your antenna. The three rod attachment is principally there for lighting protection. It is designed for rapid disbursement into the ground upon a strike on the antenna allowing less damage to the equipment. The bond is proper procedure. The radio ground should not be attached to the antenna ground it should be fed back as the green or bare wire to the main panel with the power circuit. If you install a subpanel the neutral bus should be isolated from the ground bus and the ground in the subpanel should be fed back and attached to the single point of ground in the main panel.

So on your drawing the wires shown from the radio to the antenna should go away and the wire showing a ground going from the antenna to the panel in house should go away just leaving the buried #6 bond in place. Then the ground from the radio should be run to the panel as mentioned above.

This keeps everything downstreaming to the original house ground whatever it may be. You can still surge through the ground, so it is a good thing to install a surge arrester at least in the main panel. I would also consider secondary surge protection at sensitive electronic equipment such as your computer or even a flat screen TV.

Alright let's try and get a little deeper. Sometimes a little sketch helps.

This image is very primitive grounding schematic showing the usual methods of grounding and includes the proper way to connect to the antenna. Also note that the NEC does not cover antennas and lighting protection that is found in NFPA 780 and IEEE but I don't know which section of hand.

Everything must be bonded back from the antenna prior to the bond at the first means of disconnect. As I have said before this is to provide as much dispersion of the strike into the ground, and then if you have installed a surge protector it will suppress the strike trying to make its way upstream either in the power lines or in this case the ground.

If you connect the radio or any other equipment attached to the antenna ground after the bond at the first means of disconnect, then you have bridged the surge protection and left your entire electrical system to the full force of the strike. The only other thing I could think of is that you might be thinking a shield is a ground to the radio (like the outer protection of a coax). But a shield is a drain that bleeds off outside induction and frequency interference. It should be attached to the ground on one end but is left ungrounded at the other ergo it is not a ground. If you say it is impossible to unground the radio from the antenna then you must not attach it to the building ground and make it a separately derived system.

As I have said before grounding can become a fairly complex problem because it means different thing to different people. Is it grounded or grounding, a bond, an equipment bond or ground, a counterpoise, or a signal reference grid, or a ground ring or even a shield and many others.

Personally I am just trying to help people who may not have the same skill level from getting themselves into a situation where they are creating a harm to themselves or others. Hope this helps.