I came across the claim somewhere recently that the proliferation of DP isolating switches in domestic circuits might be dangerous, because if the live contacts get welded closed, flipping the switch off will misleadingly turn the neon indicator off and disconnect the neutral but not the live.
How often does this problem actually happen?
Had it happen on cheap 13A sockets, but aren't the DP switches mean to have wider separation, so shouldn't it be less likely?
Lee
Well I had it on a servo motor contactor on a cnc lathe week before last if that counts. Switching 90 amps at 150v DC - there was a fault on the servo driver, the contacts closed, vapourised themselves but still managed to weld in the closed state on both poles! Made a very impressive bang, a smell like Frankenstein recreating life, and blew up a very expensive servo controller! Nothing left of the actual contacts - just the bar they mount on. Fortunately it's a four pole contactor with only two used so I could just use the other pair until a spare presents itself at a sensible price.
AWEM
That's not much of a risk, it would require multiple other faults before anyone got a shock. The main added risk with dp switching is that there are 2 switches that can catch fire due to poor contact, not one. I don't see any safety gain in dp.
NT
anyone got a shock. The main added risk with dp switching is that there are= 2 switches that can catch fire due to poor contact, not one. I don't see a= ny safety gain in dp.
It's about isolation, not switching.
anyone got a shock. The main added risk with dp switching is that there are 2 switches that can catch fire due to poor contact, not one. I don't see any safety gain in dp.
Its all about risks.
NT
The MK DP 13A sockets that I fitted have 3mm air gaps - the same as a 40A pull-switch.
I wondered if that was the reason for specifying the contact gap in some requirements.
Sounds like a fairly unusual situation that made this happen?
Welcome to DC land.
AB
Yes but the date difference between the original thread and now would seem to suggest there is a very low occurrence of this sort of problem. Get a mercury switch!
Brian
Perhaps a Remoaner thing to be oblivious to the fact the post you were replying to is nearly 6 years old.
The only conclusion is contacts that have welded together are a rarity.
It happens that Archibald Tarquin Blenkinsopp formulated :
Switching DC is always a problem. My caravan mover has two motors each drawing upto 90amps, switched by relays, operated by a remote control. It could be disasterous for the relays to weld up, so the relays are doubled up in series. Should one of the relays fail to release, hopefully the second one would release. The control system then goes into fault until the welded contacts are freed up. It is fairly easy to get them to weld, by inching to rapidely. It is usually easy to clear with just a smart tap on the control box, with the power off.
I used to have a computer who's tape drive DIP control relay would weld up. A similar smart tap on the IC would clear it.
whose
Go solid state? eg.
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