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Automatic disconnection of supply, and how a shock happensHow much current flows through wet hands?
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Automatic disconnection of supply (ADS)
An electric shock from equipment happens when an exposed conductive part becomes live because of a fault. For example, a pan of water boils over into an oven and makes a connection between line and the metal case.
- Automatic disconnection of supply (ADS): used almost universally in the UK.
- Double or reinforced insulation (Class II equipment): two layers of insulation, or one layer of reinforced insulation, so the outer casing can't become live.
- Extra-low voltage (SELV and PELV): for example SELV, supplied from a separated extra-low voltage (safety isolating) transformer.
- Electrical separation: supply from a separating transformer, like a shaver socket.
BS 7671 · Chapter 41 · protective measures generally permitted
ADS only works when the exposed conductive part is connected to the MET by a CPC, and the MET is connected back to the star point. You can't have ADS without that physical path. The circuit breakers, fuses and RCDs in the consumer unit need a path of low impedance. A low impedance means a high fault current, and a high fault current trips the device quickly.
Our example: fault current
Earth fault loop path = 1 Ω, voltage = 230 V I = V ÷ R = 230 ÷ 1 = 230 A of fault current → any circuit breaker or fuse in the consumer unit trips = ADS (An RCD or RCBO works slightly differently, but it still needs an earth fault loop path.)
While the fault is there, the equipment is live until the device disconnects. That might be as quick as 0.1 s, or as long as 5 s. Different circuits have different maximum disconnection times, which is the subject of another masterclass.
How the shock happens
During that short window, imagine someone is touching the live exposed part with one hand and a metal pipe with the other. The pipe goes into the ground, so it's at earth potential (0 V). You can prove that: stand barefoot on your lawn and you're touching earth, but you feel nothing, because the ground is at 0 V.
The exposed part is at 230 V and the pipe is at 0 V, so there's a 230 V potential difference. The person bridges the gap. Current flows through their arms, across the chest and heart, and into earth through the extraneous conductive part.
Dry hands
Average body resistance ≈ 1000 Ω I = V ÷ R = 230 ÷ 1000 = 0.23 A → more than enough to send the heart into fibrillation (cardiac arrest)
Wet hands
Body resistance lower, say 500 Ω I = V ÷ R = 230 ÷ 500 = 0.46 A → double the current. A child's resistance is lower still (thinner skin, smaller body).
People often say "voltage moves" or "voltage flows". Strictly, voltage is the potential difference that drives current, and it's the current through the body that does the harm. This chain is worth learning: no potential difference → no current → no electric shock.
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