I got a shock from a metal switch powering mains

Question

I've been working on an Arduino project which controls the temp of a crock pot and I wanted to incorporate this cool looking illuminated metal toggle switch. https://www.alibaba.com/product-detail/DaierTek-16mm-Round-Mounting-Waterproof-SPST_1600142436167.html.

The metal switch was used to turn on a small 12V 1.5A DC power supply from the 240V household mains, which powered an Arduino mega, which controlled a solid state relay to power the crock pot. (i.e. a positive 240V was running through the metal toggle switch).

I made sure to properly earth the metal housing of the switch, however, the actual metal lever of the switch wasn't physically touching the housing, so it could NOT be earthed. I was not using the RGB LED in the switch at the time.

The switch was also mounted onto an aluminum panel, which was earthed.

They do say the switch is rated for only 125VAC, not 240V, but they do briefly mention that the max voltage is 250VAC.

Anyway, I used the switch successfully around 50 times to turn on and off the circuit. Then yesterday when I went to turn off the circuit, I got a huge shock when I touched the lever of the switch, which tripped the RCD on my house! Caused a big muscle spasm, but I was fine. The shock only lasted a fraction of a second.

After it happened, I checked the wiring multiple times with a multimeter and everything was perfectly fine. There was no continuity between the lever and any of the switch contacts and everything (except for the lever) was properly earthed. And the live wires connected to the switch contacts weren't touching any of the metal housing.

So what may have happened? Could the distance between the metal lever and the live contact have been too small and the current jumped through the air? Would using only 125VAC really have prevented this? Is this a problem with other metal switches too? Either way, I'm sticking to plastic switches for now.

Answer 1

Despite the 'Quick Details' mentioning 'Max. Voltage: 240 V AC' and the 'Product Description' mentioning 'Rating - 20A 12VDC;15A 125VAC', there is every indication that it's a low voltage switch.

1. The description 'Illuminated Metal Toggle Switch with Wire for Car Boat' shows that it's intended for low voltage DC application in a car or a boat.

2. In the same description, '12V RGB Illuminated' indicates an LED voltage of 12 V. The 'Wiring Diagram' picture, showing the LED positive connected to the switch positive, confirms that the switch is indeed intended for 12 V DC application.

The lesson learnt from your experience would be to always use all-insulated switches and to never use metal ones, in utility mains applications.

So what may have happened?

It could be insulation failure (tracking) caused by application of high voltage to a low voltage device.

'Tracking' is the formation of a conducting path across the surface of an insulation at high voltage.

A multimeter would not reveal insulation failure as it operates at a very low voltage. The instrument intended for that purpose is known as a 'megger'.