I have drilled into an electric cable — what do I do?

The first sixty seconds, why you must not pull the drill out first, what has to happen before the wall is closed up again, and why this is one of the few jobs with no user-level fix.

Difficulty: beginner
Time: 15 min
Read: 5 min
Safety: warning

Short answer

Do not pull the drill out and do not touch it. Go to the consumer unit and switch off the main switch, or the circuit if you are certain which it is. Then unplug the drill, remove it, and call an electrician — a buried cable that has been drilled must be properly repaired or replaced, and it is notifiable work in most cases.

Drilling into a cable is common enough that it happens to careful people, and the outcome depends almost entirely on what you do in the next minute. The instinct is to yank the drill out and look at the damage. That is the wrong move, and this is one of the very few situations in home DIY where the correct answer is entirely about not touching things.

Safety

Do not touch the drill, the bit or the wall if the drill is still in the hole and you have any reason to think a cable is live. Do not pull the drill out. Switch the power off at the consumer unit first, from a position where you are not touching the tool. If anyone has received a shock, has burns, or feels unwell, ring 999 — an electric shock can cause problems that appear later. A damaged buried cable must be assessed and repaired by a qualified electrician; it is not a DIY repair and taping it up is not an option.

Step by step

Let go of the drill if you have not already.If the RCD has tripped and the lights are off, the circuit is already dead. If it has not, the cable may still be live and the drill is in contact with it. Step back and do not hold the tool.
Do not pull the drill out yet.The bit may be bridging conductors or sitting in the cable's insulation. Withdrawing it can create an arc, complete a different fault path, or drag copper strands out through the plaster. It stays where it is until the power is off.
Switch off at the consumer unit.Main switch off is the safest and quickest action if you are at all unsure which circuit it is. If you know the circuit with certainty, switch that breaker off instead. Do this before touching anything else.
Check the power really is off.Try the lights or a socket on that circuit, or use a socket tester or a voltage indicator if you have one and know how to use it. Never assume a switched breaker has isolated what you think it has, particularly in a house with unlabelled or historically modified circuits.
Unplug the drill and remove it.With the supply off, unplug the drill from its socket, then withdraw the bit from the wall. Keep the drill isolated until it has been looked at — a bit that has been through a live conductor can damage the tool.
Look after anyone who got a shock.Even a shock that seemed minor warrants medical advice, because electrical injury can cause delayed cardiac effects and internal burns that are not visible. If there is any loss of consciousness, chest pain, burns or an abnormal heartbeat, ring 999.
Leave the circuit off and call an electrician.Do not restore power to the circuit. A cable with a hole in it may look like a small fault and may have damaged insulation over a much larger area, and it can arc inside the wall out of sight. Repair means either a proper enclosed joint in an accessible position, or replacing the whole cable run.
Tell the electrician exactly what happened.Where the hole is, how deep the bit went, which circuits went off, whether an RCD tripped, and whether anyone was shocked. It is diagnostic information and it saves the electrician chasing a fault they could have been told about.
Do not make good until it has been repaired and tested.Filling the hole hides the damage. The wall gets closed up after the cable has been dealt with and the circuit has been tested, not before.

Tips

If the RCD tripped, that is the protective device doing exactly its job, and it is a strong argument for having modern RCD protection on every circuit in a house where anyone drills walls.
An RCD that will not reset is telling you the fault is still present. Repeatedly resetting it is both futile and unwise.
Photograph the hole and the surrounding wall before anything is disturbed. It helps whoever repairs it and it may matter for an insurance claim.
A drilled cable in a rented property is the landlord's problem to fix, but tell them immediately rather than after the wall has been filled.

If it doesn't work

Bang, flash and the lights went out

Cause: The bit shorted line to neutral or line to earth, and the breaker or fuse cleared the fault — Fix: Leave everything switched off. This is the best of the possible outcomes because the protection worked. Do not reset anything until the cable has been repaired.

Nothing happened at all

Cause: The bit may have nicked the sheath without reaching a conductor, or the circuit may have been switched off at the time — Fix: Still isolate and still investigate. Damaged insulation with no immediate symptom is the most dangerous version, because it can arc later, unattended, inside a wall.

The RCD trips again as soon as it is reset

Cause: The damaged cable is still faulting to earth — Fix: Leave it off and call an electrician. The RCD is not faulty; it is detecting a real fault that has not gone away.

A burning smell but the power is still on

Cause: A high-resistance fault heating up inside the wall without drawing enough current to trip anything — Fix: Switch off at the main switch immediately and call an electrician. This is the scenario that starts fires and it does not resolve itself.

Questions people ask

Can I repair the cable myself with a junction box?

No. Adding a joint to a buried cable is not permitted work for a householder in most circumstances, and any joint must be in an accessible position or of an approved maintenance-free type. Beyond the regulations, the practical problem is that you cannot see how far the damage extends inside the sheath.

Is a hole in the outer sheath serious if the wires look fine?

It has to be treated as serious. Moisture and plaster reach the conductors through the sheath, and the inner insulation may be nicked in a way you cannot see. Have it assessed rather than assumed.

Why did nothing trip when I hit it?

Several possibilities: the circuit was off, the bit contacted only the earth conductor, the damage was to insulation without a conductive path, or the fault current was not enough to operate the protective device. None of them means the cable is undamaged.