Electrical – Can a CAFCI be double-lugged

Pick an electrical code

Pick an electrical code and follow it. Pick one where your wiring system is normal. If your power is 110/220V split-phase (220V with the neutral center-tapped with 110V on each side), then follow a North American code. If your power is all 220V between neutral and "hot", then follow an EU code.

On split-phase, neutral is serious business

One of the scary things to be found in the North American split-phase system is if you lose a neutral wire. The 120V circuits become imbalanced, and half of them are more than 120V and the other half are less (but they total 240V). And they teeter-totter back and forth as devices blow out until eventually everything smokes. Losing neutral is serious business, and if you can't get reliable neutral from the building owner, you may have to use the outer 240V(220V) lines and manufacture your own 110V service with your own transformer.

This is not an issue if you're supplied single phase 220V from a European style system, if you lose neutral you lose power.

Any shock is deadly shock

People often get "small" shocks and go "well, that's not dangerous". Yes, it is. Remember, electricity needs to complete a circuit. It flows when two things are connected. You made a solid connection with a deadly amount of current from the TV/fridge, however, your connection back to ground/earth wasn't very good, so not much current could flow. Different deal if there's water on the floor or you happen to be touching the sink.

So any appliance that gives you a shock, is a 5-alarm fire and needs to be Dealt With Right NOW before someone else has a "better" connection!

GFCI's are great for shock hazard reduction

A GFCI (RCD in Europe) compares the current flow on the "hot" wire, to the current flow on the "neutral" wire. In normal operation, they are exactly the same; that's what it means to complete a circuit. Current seeks to follow all possible paths, regardless of resistance. If current finds an alternate path (like through you), some of it will go that way, and the GFCI will see that the "hot" flow and the "neutral" flow is not the same, and it will trip. This means your shocking experience won't last for very long.

Note I don't mention ground at all. Like the honey badger, GFCI's don't know or care about ground. Obviously, ground may be part of the fault path, but the GFCI doesn't care about that, it cares that the current is NOT the same on hot and neutral. This means GFCI's do not need ground at all. They are often used to provide safety when a ground is not available - in fact, they provide far better safety when dealing with plastic-chassis equipment, since they are actually looking for electrocution situations.

GFCIs do not provide equipment grounding, however, and sometimes equipment cares about that. Particularly, sensitive electronic equipment which does not like ESD (electro-static discharge, or that zap you get when you shuffle your feet across a carpet then touch a doorknob). That's a ground for a different kind of reason: not personnel protection, but equipment protection. That ground does not need to be tied to the electrical system "neutral". An example of that would be the microcomputer inside a large machine tool connected to 480 3-phase (plain) "delta" power, all 3 wires of which are hot. (let's say that's protected by GFCI so a hot-chassis fault in the motor won't kill the operator). The computer is powered by a small transformer off 2 legs of the 480 delta, which isolates it from everything. The computer needs a ground to give ESD somewhere to go.

Get a better ground than concrete

Concrete does have some conductivity, but I wouldn't rely on it for a long distance. Anyway, "9 storeys tall" means not wood. 1940 probably means steel not concrete. The steel frame is most likely your best source of an equipment ground. You can use reinforcing rod in a building designed for this but they weren't really thinking about that in 1940.

Do Not tie neutral to ground!

Neutral must be tied to ground in exactly one location: the main panel, as in the building's main panel. That's because Neutral Is Not Ground, and there is a voltage difference between neutral and ground. If you tie neutral and ground together in a second location, that voltage difference will result in current flowing, and potentially a surprising amount, since it could involve the whole building's current return! This means ground is handling current on a regular basis, which it's not meant to do. This also breaks any GFCI protection upstream.

You only tie neutral to ground in the main service, which is directly next to the supply transformers. This is necessary since a supply transformer isolates the input power from the output power. It would "float" (at indeterminate voltage from ground) unless you peg it to ground with a ground strap/bond.

Anytime you have current flowing on a neutral-ground bond, that is a very bad thing and needs to be corrected immediately.

Sounds like a ground fault or arc fault is being detected on the circuit, remove the load and see if it will turn on. If it turns on without the load connected you have a problem on that branch circuit. Make sure if you install 3 prong outlets to label them GFCI protected no equipment ground.