Sinan

The device does not need a ground wire to work, and understanding why makes the rest obvious.

A ground fault interrupter compares the current flowing out on one conductor with the current returning on the other. In normal operation they match. If some current is escaping by another route — through a person, through a damp wall — the two no longer balance, and the device cuts the supply within milliseconds at a very small imbalance.

Notice that nothing in that description involves a ground wire. It is measuring a difference between two conductors it already has.

So for protection of people against shock, which is the thing that hurts you, this genuinely works without a ground. Many electrical codes explicitly permit exactly this replacement in ungrounded systems, on the condition that the outlets are labelled to say there is no equipment ground. Those labels usually come in the box, and fitting them is part of the job rather than an optional extra.

What you do not get is worth listing precisely, because this is where people over-estimate the substitution.

• No path for fault current. With a proper ground, a live conductor touching a metal appliance case dumps current to ground and trips the breaker immediately. Without one, the case simply sits energised until somebody touches it — at which point the GFCI saves them, but the fault was live until then.
• No surge path. Surge protectors and many filters rely on a ground to divert energy. A surge strip plugged into an ungrounded outlet provides much less than its label suggests.
• No reference for sensitive equipment. Some electronics and audio equipment behave badly without one.
• Not everything is covered downstream. Protection extends to outlets fed from the protected one, and only if wired correctly.

So: good protection for people, absent protection for equipment. That is the honest summary.

You have essentially answered it: a correctly sized cord in good condition really would be electrically fine. The warning is written for the general case, and the general case is bad for reasons that go beyond the conductor size.

What makes heaters specifically dangerous with cords:

• They draw near the maximum, continuously. Most appliances are intermittent or light. A heater pulls close to the circuit's rating for hours without pause. That is the worst possible duty for a marginal connection, because heat accumulates rather than dissipating between uses.
• The failure point is usually the connection, not the wire. A slightly loose plug in a socket, or a worn contact in a cord end, has resistance. Resistance times a large continuous current is heat, right at a plastic connector. This is how these fires start, and a heavy cord does not protect against it.
• Cords get covered. Under a rug, behind furniture, coiled up. A coiled cord carrying a heavy continuous load cannot shed heat and gets substantially hotter than the same cord laid out.

The cord in the wall does not have plug connections, is not coiled, and is not walked on.

Turn off the breaker for that circuit now, and leave it off until it is repaired. That is the whole answer and everything else is detail.

What you have described is not a minor fault. The lights changing as you move the nail means the nail is carrying current, or is intermittently making and breaking a live connection. Both are serious:

• The nail is energised. It is metal, it is in a wall, and it may be touching plaster, a fixing, or whatever you were hanging. Anyone who contacts it while grounded is at real risk.
• The cable is damaged and thinner where it was struck. Current squeezed through a reduced cross-section produces heat, inside a wall cavity, next to timber and insulation.
• An intermittent connection arcs. Arcing is how fires start in walls, and the fact that the state changes as you move the nail means it is arcing every time it shifts.

Do not pull the nail out with the circuit live. Switch it off at the panel first, and label the breaker so nobody helpfully turns it back on.

One caution about the project rather than the wire.

Check what the device actually needs before you commit — voltage, current, and whether it wants regulated supply. Network terminals and similar equipment are sometimes fussier than they look, and some expect a supply that can deliver a surge well above its steady draw.

Also check whether the battery backup you are planning to use outputs a waveform the original adapter is happy with, if you are keeping the adapter in the chain. Cheap backup units produce an approximated waveform that some supplies dislike, and the failure shows up as an intermittent reboot months later rather than immediately.

There are two reasons and the second is the one that makes it a rule rather than a preference.

Reusability. A conduit system is meant to be a raceway you can pull new conductors through later — that is much of the point of using it rather than cable. If the wires were laid in as the run was assembled, there is no guarantee the path is actually pullable. A joint that was fine to slide over a wire may have an internal edge or an offset that will not let anything through afterwards. You discover that in ten years, when somebody needs a new circuit and finds a conduit that cannot be used as one.

Damage you cannot see. Assembling around conductors means sliding fittings over insulation, threading joints beside it, and possibly cutting or reaming with wire present. Every one of those can nick insulation in a place that is now permanently enclosed. A nick inside a metal raceway is a fault waiting for vibration or heat, and there is no way to inspect it.

Pulling afterwards, with proper lubricant and a smooth complete raceway, is a controlled operation where damage is far less likely and far more likely to be noticed.

You have walked past a great many of them, and this is close to universal — it is one of the first things you notice when you start looking deliberately.

Three things are working against you:

They detect you first. Snakes are exquisitely sensitive to ground vibration. A person walking is an enormous signal, and the response of nearly every species is to leave or to freeze before you are anywhere near. You are being avoided, continuously, by animals you never register.

They are built not to be seen. Camouflage in snakes is extraordinary, and it is specifically tuned to the substrate they live on — leaf litter, bark, gravel. A motionless snake on the right background is genuinely invisible at two metres, and freezing is the default response of many ambush species.

They are mostly not where you are looking. Under things, in things, and largely active at times of day when you are not there.

Surveys that use proper methods — cover boards, transects, trained observers — routinely find densities that astonish people who have walked the same ground for years.

The other reliable approach when acetone is unavailable is time plus mechanical patience: repeated soaking in warm soapy water, gentle rolling, and an oily substance to work into the edge of the bond as it loosens.

Skin renews itself. Even with no solvent at all, a bond on skin releases within a day or two as the outer layer sheds. So the worst realistic outcome is inconvenience rather than anything permanent, which is worth knowing because panic is what makes people pull.

Also: the adhesive itself is broadly the same chemistry as the medical version used to close wounds, which is why this is a nuisance rather than a poisoning.