Simple AC Mains switch (power switch) question.

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jwhmca

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Jan 28, 2009
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Pros and cons of switching both the Hot and Neutral (DPDT), as opposed to just the Hot?
 
One "pro" I have found as I research this is "polarity" protection. If the power cord is flipped it still removes the Hot side. I haven't found any cons yet.

I would of course love to hear what you have to say John...
 
In the USA, I know that for AC power distro within a building (ie, the wiring to/from the breaker box to wall outlets, etc) requires a "solid neutral" per the National Electric Code.  Further, Underwriters Laboratories definitely allows (requires??) only the "hot" leg to be switched within a piece of gear that receives the UL approval sticker.

Bri
 
Brian Roth said:
In the USA, I know that for AC power distro within a building (ie, the wiring to/from the breaker box to wall outlets, etc) requires a "solid neutral" per the National Electric Code.  Further, Underwriters Laboratories definitely allows (requires??) only the "hot" leg to be switched within a piece of gear that receives the UL approval sticker.

Bri

Our TÜV guy tells us that equipment destined for countries that require that certification must switch both hot and neutral.

-a
 
Perhaps UL has changed their rules in recent times, but I've been inside a LOT of UL listed electronics gear that had only the hot lead switched, and the neutral "solid". 

Bri

 
Brian Roth said:
In the USA, I know that for AC power distro within a building (ie, the wiring to/from the breaker box to wall outlets, etc) requires a "solid neutral" per the National Electric Code.
Interesting! In France and Monaco, circuit breakers sense live only but must break both live and neutral.
Further, Underwriters Laboratories definitely allows (requires??) only the "hot" leg to be switched within a piece of gear that receives the UL approval sticker.
Some other countries specifically request both poles to be switched, I think Korea is one.
I believe in the end, the only reason is getting acceptance in some countries.
 
Switching both life/neutral is mandatory in wide parts of europe, FI Germany, Italy, Spain, Austria, Netherlands, .. that use the "Type F" CEE 7/4 Schuko plugs. Reason is, these plugs can be plugged into the wall outlet in either direction.
schuko_plg_skt.png
 
Without getting into what code rules apply. 
If both the Hot and Neutral  (or for that matter the Hot & Hot) are switched, then the switches must be interlocked.
The Neutral should never be fused.
 
kepeb said:
isn't the neutral hooked up to earth potential at source anyway?
You may like to measure the voltage between earth and neutral at your own house.  If you want some fun, put an ammeter instead of voltmeter between earth & neutral.  ;D

OK.  I shouldn't have said that.  Don't try this at home.  8)
 
ricardo said:
kepeb said:
isn't the neutral hooked up to earth potential at source anyway?
You may like to measure the voltage between earth and neutral at your own house.  If you want some fun, put an ammeter instead of voltmeter between earth & neutral.  ;D

OK.  I shouldn't have said that.  Don't try this at home.  8)

There should not be currents between them unless something is broken. Earth at least here in Finland is a quite literal earth: a +25m meter thick copper cable dug deep in the ground. Both neutral and safety earth are joined to this fat copper earth somewhere in the main electricity box. Condo's are more complex, but the main idea should still be the same.
 
Harpo said:
Switching both life/neutral is mandatory in wide parts of europe, FI Germany, Italy, Spain, Austria, Netherlands, .. that use the "Type F" CEE 7/4 Schuko plugs. Reason is, these plugs can be plugged into the wall outlet in either direction.
schuko_plg_skt.png

It makes sense to always switch both live and neutral for safety reasons. Sure the IEC C14 plug that we nearly always use for DIY has the sides well defined, but our north European Schuko is ambiguous. Also, transformers do not care.

A 120/230VAC switch is also a good idea and usually very cheap and easy to implement. You might not travel, but someday you may wish to sell that work of yours to another country.
 
I makes sense to observe spacing and insulation rules for all mains wiring. Breaking the circuit for either leg will de-enegize the transformer primary.

There are also general rules for locating fuse wiring in the hot leg, so an open fuse will stop mains voltage at the fuse holder.

JR
 
JohnRoberts said:
There are also general rules for locating fuse wiring in the hot leg, so an open fuse will stop mains voltage at the fuse holder.

But again, quite impossible for the north European Schuko.

(somewhere on this forum maybe 5 years back lies my "doh!" moment discovery of this ambiguity. I was routing mains switch to front panel and it was causing some hum to the audio circuits. It occurred to me I should route only neutral to the front panel, and hum was gone. Some days later the hum was back when I had the mains cord plugged another way around. The working solution was of course good shielding for the wire and routing them both to the switch for safety.)
 
Kingston said:
JohnRoberts said:
There are also general rules for locating fuse wiring in the hot leg, so an open fuse will stop mains voltage at the fuse holder.

But again, quite impossible for the north European Schuko.

(somewhere on this forum maybe 5 years back lies my "doh!" moment discovery of this ambiguity. I was routing mains switch to front panel and it was causing some hum to the audio circuits. It occurred to me I should route only neutral to the front panel, and hum was gone. Some days later the hum was back when I had the mains cord plugged another way around. The working solution was of course good shielding for the wire and routing them both to the switch for safety.)

Yes good advice for Euro zone readers, but the OP who is wiring up a switch is located in the USA.

I am old enough to recall when US 2 circuit power plugs and outlets were symmetrical, but for quite some time, even the 2 circuit plugs in the US have different sized blades to establish a polarity. Of course it is not wise to rely upon this for human safety. Treat both mains leads as if they were hot for spacing and insulation considerations. 

JR
 
Indeed, here in the USA, the old. so-called "Edison socket", a two pronged device with two equal sized blades on the plug, was the standard for many years.  Newer sockets and plugs now have a wider blade for the neutral and a narrower blade for the hot, along with the third "round" grounding pin.

That does NOT mean that a novice electrician's apprentice, tasked with the job of installing the wall outlets because it is "above the pay grade" of the master electrician, will install the outlets correctly.  As I've been sloooooooooooooooowly remodeling my house, built in 1970 and hence equipped with 'polarized' outlets with the third  grounding pin, I've found perhaps 25% of the original outlets were miswired....hot on the wide blade, neutral on the narrow.

Based upon comments in this thread, then perhaps both wires (hot and neutral) need to be switched, AND both wires should have a fuse, just to cover the errors found in typical USA wiring systems.  I would suggest a third fuse for the round/grounding pin, but I've yet to see such a total screw-up where that third pin was actually connected to the hot leg from the mains.  But, anything is possible.....

Bri
 
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