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?

On 14/12/2012 14:26, Adam Funk wrote:
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

On Fri, 14 Dec 2012 15:57:04 +0000, Lee wrote:

On 14/12/2012 14:26, Adam Funk wrote:
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

The MK DP 13A sockets that I fitted have 3mm air gaps - the same as a 40A
pull-switch.
--
Peter.
The gods will stay away
whilst religions hold sway

On 2012-12-14, PeterC wrote:

On Fri, 14 Dec 2012 15:57:04 +0000, Lee wrote:

On 14/12/2012 14:26, Adam Funk wrote:
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

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.

"Adam Funk" wrote in message ...

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?

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

On 2012-12-14, Andrew Mawson wrote:

"Adam Funk" wrote in message ...

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?

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.

Sounds like a fairly unusual situation that made this happen?

On Fri, 14 Dec 2012 21:33:16 +0000, Adam Funk
wrote:

On 2012-12-14, Andrew Mawson wrote:

"Adam Funk" wrote in message ...

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?

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.

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

--
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This newsgroup posting comes to you directly from...
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"Archibald Tarquin Blenkinsopp" wrote in
message ...
On Fri, 14 Dec 2012 21:33:16 +0000, Adam Funk
wrote:

On 2012-12-14, Andrew Mawson wrote:

"Adam Funk" wrote in message ...

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?

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.

Sounds like a fairly unusual situation that made this happen?

Welcome to DC land.

AB

On 08/06/2018 06:09, Archibald Tarquin Blenkinsopp wrote:
On Fri, 14 Dec 2012 21:33:16 +0000, Adam Funk
wrote:

On 2012-12-14, Andrew Mawson wrote:

"Adam Funk" wrote in message ...

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?

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.

Sounds like a fairly unusual situation that made this happen?

Welcome to DC land.

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 :
Welcome to DC land.

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.

On 08/06/2018 09:50, Harry Bloomfield wrote:
It happens that Archibald Tarquin Blenkinsopp formulated :
Welcome to* DC land.

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.

Go solid state?
eg.

https://www.ebay.co.uk/itm/100A-High...-/172015803320

On Friday, December 14, 2012 2:26:14 PM UTC, Adam Funk wrote:
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?

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

On Dec 14, 7:10*pm, wrote:
On Friday, December 14, 2012 2:26:14 PM UTC, Adam Funk wrote:
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?

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

It's about isolation, not switching.

On Friday, December 14, 2012 8:29:59 PM UTC, harry wrote:
On Dec 14, 7:10 pm, wrote:
On Friday, December 14, 2012 2:26:14 PM UTC, Adam Funk wrote:

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?

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.

It's about isolation, not switching.

Its all about risks.


NT