Touching energized microwave oven body

Assume a 230 V supply whose neutral is earthed at the transformer, and that the live wire inside the oven has touched its metal body because of insulation failure.

 L ─[MCB]──┐
           │ fault
       ┌───┴───┐
       │ oven  │──── earth wire (if present)
       │ body  │        │
       └───┬───┘        │
         person         │
           │            │
 ══════════╧════════════╧══ earth
 N earthed at transformer

Unearthed condition (no earth wire)

• The oven body rises to about 230 V with respect to earth.
• The person touching it becomes the only path to earth. With body plus contact resistance of about 1000 Ω, current ≈ 230/1000 = 230 mA.
• Currents above about 30 mA through the body can cause muscle spasm, inability to let go, and ventricular fibrillation, leading to death.
• The current is too small to blow the fuse or trip the MCB, so the body stays live and the danger continues for anyone who touches it.

Earthed condition (body connected to a good earth)

• As soon as the fault occurs, a large fault current flows through the low-resistance earth wire back to the transformer neutral (for example, with total loop resistance of about 2 Ω, the current is over 100 A).
• This current trips the MCB or blows the fuse within a fraction of a second, so the oven is disconnected, often before anyone touches it.
• If the person touches it during this short time, the body is in parallel with the very low earth path, so only a small current passes through the person. With an RCCB (30 mA) the supply is cut in about 30 ms.

Conclusion: the unearthed oven is a serious shock hazard; the earthed oven makes the fault safe by clearing it quickly. This is why every appliance with a metal body must use a 3-pin plug with the earth pin connected.