Two Way Switch Disaster or DIY = Death In Yellow

[bernardgreen]

Friends bought an ancient cottage in June. Previous owner had it re-wired just before putting it on the market.

They have replaced switches on the two way light on the stairs at least twice since moving in. Also experience frequent trips of the 6 amp MCB when switching the light:-

Discovered the circuit to be

Live to L1 in both switches

Neutral to L2 in both switches

Common of one switch to one side of the lamp

Common of the other switch to the other side of the lamp.

which means both sides of the lamp can be live when it is switched off.

Bernard

Sharnbrook

 

[Steve]

were any certificates offered or produced? If not, the previous owner can be prosecuted IIRC. That'll teach him to mess about with things he doesn't know about.

 

[Spark123]

It just sound so wrong, the switches are not meant to deal with switching between live and neutral hence is a good reason why they will not last. Sounds like it requires immediate remedial work.

 

[plugwash]

i suspect the reason for the trips is that the switches probabally aren't built to gaurantee break before make.

 

[bernardgreen]

Remedial work finished, as luck would have it a reasonable easy task as all involved cables meet in a junction box.

This is my theory for switch failures

The switch is not a fast snap action so some arcing occurs when ever the lamp is turned off. This breaking circuit arc continues long enough till the moving contact touches the other fixed contact. As this fixed contact is 230 volts different from the other fixed contact the arc now is across the 230 volts between the two fixed contacts and continues until the MCB opens.

In the correctly wired circuit the breaking circuit arc may remain until the moving contact reaches the other contact but this contact is not connected to any voltage so the arc eventually dies.

Bernard
Sharnbrook

 

[croydoncorgi]

This is the same phenomenon that causes MCBs to open when a lightbulb fuses. A plasma caused by arcing can conduct a LOT of momentary current!