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As awesome as knife switches are..

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Thomas W

Sun Feb 23 2014, 11:14PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
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Posts: 1276

As awesome as knife switches are. What are legal places and such to use them.

Often i see things like this on ebay:

Why are they sold? and who and where can you use them?

Im rather sure legaly you cant use such an exposed device on a wall, they are designed and rated for powers far higher then i would want to try using such a style of switch for.

If an slightly lacking skilled person was to use a 200A knife switch like the second one with a large motor of similar rating.. the potentual for arc flash or such.. well it scares me.

Proud Mary

Mon Feb 24 2014, 12:32AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I think they are sometimes used as a sort of 'disconnect of last resort' - which is why some of them have padlock lugs on them - rather than as an on-off switch e.g. if the switch is in the padlocked open state, then everyone can clearly see and be 100% certain that no current flows beyond that point.

I have a small antique porcelain knife switch from the 1920s, which has Ã† (aerial) and E (Earth) terminals embossed upon it, so that an antenna can be connected safely to ground when not in use, or during thundery weather.

Shrad

Mon Feb 24 2014, 09:08AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780

they are meant to securely isolate a portion of your circuit out of the power grid

some will have hardware for a lock or even an integrated keylock to prevent someone to accidentaly power on the circuit when people work on it

high power ones have a crank used to charge a spring which will make or break contact at balistic speed to avoid spark pitting the contact areas (usually starting at 15kV 500mA and more)

Thomas W

Mon Feb 24 2014, 09:22AM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
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Posts: 1276

As cool as they are, ive alwaysd been a bit scared of them, mainly because when you pull the knifeswitch open, the contacts that are live are rather easy to access.

I don't think you could use the 200A orange one at all for many things considering the terminals arn't covered, it looks like a huge safety risk compared to the nice steel enclosure switches.

Mads Barnkob

Mon Feb 24 2014, 09:39AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I never saw a knife switch installed in Europe and I did see quite a few cabinets travelling around for 2,5 years.

What is used in Europe is the reparation switches that have a high IP class for installation in the production and are mechanically constructed for lock securing.

For securing large switches or supply lines, the procedure is that you remove the fuse holder from the supply cabinet, along with fuses, put up a sign and lock the cabinet.

Thomas W

Mon Feb 24 2014, 09:57AM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

I am planning, however to find a really nice looking knifeswitch to put on my wall, using a low voltage source (5v) through thick looking wires going to my control cabinet, only connected to a relay connecting to a 100A contactor i have for the main power to my 32A sockets in my room.

Mads Barnkob

Mon Feb 24 2014, 11:28AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Thomas W wrote ...

I am planning, however to find a really nice looking knifeswitch to put on my wall, using a low voltage source (5v) through thick looking wires going to my control cabinet, only connected to a relay connecting to a 100A contactor i have for the main power to my 32A sockets in my room.

It is a nice idea, but you still have the risk of a malfunction in the control cabinet so that mains power can make it into the low voltage circuit and not necessarily trip any breakers.

Maybe put it in a "IN CASE OF" glass frame?

Sulaiman

Mon Feb 24 2014, 02:42PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

When I first came across knife switches
they were just about the only kind of high current switch around,
many had two small pancake coils in series with the contact,
one either side of the contacts.
When opening the contacts the arc got blown out of a chute !
(Fleming's left hand rule)

Even the common miniature circuit breakers can be locked
(search for mcb lock/lockout)
as above, I think these are for 'guaranteed' isolation,
you shouldn't really rely on an mcb for safety, especially if there are contaminants around.

I like the look of open/simple knife switches but not safe for hv

For effect you can get a push-to-open (safety) switch with a big red dome
' PANIC ! ' . . . . . . a 'STOP' switch with the label changed
in which case it really should as directly as possible, disconnect all power.
usually in series with main contact breaker / contactor coil, hence a latching push-to-open switch also allows multiple series switches and covers broken/disconnected cables etc.

Thomas W

Mon Feb 24 2014, 06:20PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Mads Barnkob wrote ...

This is a hobbiest thing... and i find it rather hard to imagine somthing happening to cause such a incident as mains getting into the switch, but i shall take your words into accound

Proud Mary

Mon Feb 24 2014, 07:00PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The US Department of Labour Occupational Safety and Health Administration (OSHA) has regulations concerning the use of knife switches, for example:

1910.305(c)(1)

Single-throw knife switches. Single-throw knife switches shall be so placed that gravity will not tend to close them. Single-throw knife switches approved for use in the inverted position shall be provided with a locking device that will ensure that the blades remain in the open position when so set.

1910.305(c)(2)

Double-throw knife switches. Double-throw knife switches may be mounted so that the throw will be either vertical or horizontal. However, if the throw is vertical, a locking device shall be provided to ensure that the blades remain in the open position when so set.

1910.305(c)(3)

Connection of switches.

1910.305(c)(3)(i)

Single-throw knife switches and switches with butt contacts shall be connected so that the blades are deenergized when the switch is in the open position.

Source:

â€¢ Part Number: 1910
â€¢ Part Title: Occupational Safety and Health Standards
â€¢ Subpart: S
â€¢ Subpart Title: Electrical
â€¢ Standard Number: 1910.305
â€¢ Title: Wiring methods, components, and equipment for general use.

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