Switching the neutral

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Switching the neutral

Postby Pikey » Mon Jul 09, 2018 10:59 am

Some 30 years or more ago when I was learning about mending white goods, we had occasion to look at a faulty INDESIT washing machine. The tutor said that "for some odd reasons INDESIT switch the neutral, not the live. It doesn't matter much because it all works out fine, but you should remember this when working on INDESITS "

NOTE - I am speaking of UK electrics - we have a single phase 240 volt sine wave and the "hot wire" is "Live" - I sort of understand US Electrics and this would equate to a 110 volt system on yours. The "4 wire" system you use for 240 (220?) is Not known well enough by me to coment on in this post!

Well I had an electrical fault in my old tea urn still t'other day and when I took the cover off, the fault was simple enough - a spade connector had pulled off the incoming wire to the switch. (Loose cable clamp! ) = However I found that the "live wire" to the switch had been getting hot and the insulation had gone hard and begun cracking. This was MY wiring Adaption in fitting a switch in the live circuit.

When I looked deeper, the old system had the remains of a simple on-off switch in the neutral, which was wired in parallel with a neon "on indicator lamp" (which many nowadays mistakenly think of as LEDs) I was surprised that such a thin wire could be used in the output side of the circuit - until I thought about it.

Clearly when we put 2500 watts into a heating coil through a switch, we need a heavy duty switch. However, when the voltage exits the heater, there is a small ampage, because most of the curent has gone into heating the wash ! Thus a smaller and much cheaper switch can be used if we switch the ouput side and the internal resistances of the switch are far less critical !

Heat generated equals Current squared x Resistance !

Oh and if you gather up some old "Neons" in your travels through scrap electric appliances, they are really handy "on-off" lights.

I rewired the thing to remove the Live side switch and instead put in a new one to switch the neutral.

SOmething to bear in mind folks when you think about "Converting or adapting to electric".

[Edit - During this thread it has been identified that the "Current" remains constant - but the voltage drops - ie 240 volts at say 10 amps flows in and close to 0 volts flows out supposedly at 10 Amps - I see no reason to debate this point at length it is the POWER consumption Watts equals Volts x Amps which matters and we have "lost" 240 Volts x 10 Amps = 2400 watts in heat. therefore the potential of the neutral side of the circuit to produce further heat and damage our circuitry is way reduced if we switch the neutral instead of the live - some "safety" issues are also discussed within the thread ]
Last edited by Pikey on Tue Jul 10, 2018 4:44 am, edited 1 time in total.
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Re: Switching the neutral

Postby zed255 » Mon Jul 09, 2018 12:06 pm

Pikey, the live and neutral current will be the same. Current is not 'used up' by the element, but rather it is the current flowing through (in and out) the resistance of the element that generates heat. The amount of current that flows through the resistance of the element is determined by the voltage applied and the amount of resistance the element possesses.

In some cases, such as reactive loads ( i.e. capacitive and inductive) like motors and cuircuits containing capacitors, low side or neutral switching can increase the life of a switch by reducing the likelihood of arcing out the contacts. That is an engineering decision.

In general practise I would encourage switching the live to make a build de-energized when 'off', otherwise the electrical potential is present everywhere in the build only having no path 'out'. Of course we all unplug stuff before working on it, right?

I'm not a licensed electrician but I do have a formal education in electronics. Your P= I^2*R is correct but all current flowing into something will flow out. It is never lost or used up. Power can also be calculated using P=V^2/R, or P=V*I, depending on what you know or can measure.
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Re: Switching the neutral

Postby jonnys_spirit » Mon Jul 09, 2018 12:33 pm

Easy napkin math. Round the #’s to make it easy then guestimate. 120V = 100. 8 ohms = 10 ohms. One 100W light bulb pulls 1A. 25 of em pops the circuit (20A) or is pretty close and is about the same as 2500W of power (heat).

E=I*R (elvis is rotting)
P=I*E (duhhhh pie?)

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Re: Switching the neutral

Postby Pikey » Mon Jul 09, 2018 12:41 pm

zed255 wrote:Pikey, the live and neutral current will be the same. Current is not 'used up' by the element, but rather it is the current flowing through (in and out) the resistance of the element that generates heat. The amount of current that flows through the resistance of the element is determined by the voltage applied and the amount of resistance the element possesses.

In some cases, such as reactive loads ( i.e. capacitive and inductive) like motors and cuircuits containing capacitors, low side or neutral switching can increase the life of a switch by reducing the likelihood of arcing out the contacts. That is an engineering decision.

In general practise I would encourage switching the live to make a build de-energized when 'off', otherwise the electrical potential is present everywhere in the build only having no path 'out'. Of course we all unplug stuff before working on it, right?

I'm not a licensed electrician but I do have a formal education in electronics.


Your P= I^2*R is correct but all current flowing into something will flow out. It is never lost or used up. Power can also be calculated using P=V^2/R, or P=V*I, depending on what you know or can measure
.


Now that's how I saw it too ! -

Think of it like this -

if 2.5 kw flows in and 2.5 kw flows out again - Why do we need 2.5 kw ?

And If 2.5 kw flows in and 2.5 kw flows out - We are getting completely free cooking of our boiler - No ?

Certainly the ENERGY flowing in = THe ENERGY flowing out ! but almost All of that energy is converted from electrical to HEAT !

So the output electrical energy will be way lower than that flowing in !

That misunderstanding, is precisely why I wrote this post ! - THe loss of current is far from "Intuitive" for the layman !

Thanks for your reply though, it shows I was not the only one thinking in those terms 8)
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Re: Switching the neutral

Postby Yummyrum » Mon Jul 09, 2018 3:11 pm

Pikey , I'm not sure if you actually believe this or you are just throwing up some crazy food for thought but for those that may be reading this it's nonscence . Hey I have no problems with banter and thinking and discussing things but we must not suggest bad practices as someone will try and copy it and likely suffer for it .

So for the record here:
Do not use a small switch in the Neutral line .
Do not use smaller size wiring in the Neutral line .

zee255 comments are correct . The same current flows in the live and the Neutral .

It is this basic princpal that allows earth leakage detectors / residual current detectors , what ever you call them in your country to work .
If they detect that less current flows in the Neutral line than is in the Live wire then the "extra" current is flowing to ground , quite posibly through a person touching something live and the trip curcuit operates to kill the power .

Where ever possible high side ( in the Live or Hot wire ) should be carried out as a safety reason .
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Re: Switching the neutral

Postby steelmb » Mon Jul 09, 2018 3:30 pm

zed255 wrote:Pikey, the live and neutral current will be the same. Current is not 'used up' by the element, but rather it is the current flowing through (in and out) the resistance of the element that generates heat. The amount of current that flows through the resistance of the element is determined by the voltage applied and the amount of resistance the element possesses.

In some cases, such as reactive loads ( i.e. capacitive and inductive) like motors and cuircuits containing capacitors, low side or neutral switching can increase the life of a switch by reducing the likelihood of arcing out the contacts. That is an engineering decision.

In general practise I would encourage switching the live to make a build de-energized when 'off', otherwise the electrical potential is present everywhere in the build only having no path 'out'. Of course we all unplug stuff before working on it, right?

I'm not a licensed electrician but I do have a formal education in electronics. Your P= I^2*R is correct but all current flowing into something will flow out. It is never lost or used up. Power can also be calculated using P=V^2/R, or P=V*I, depending on what you know or can measure.


Very diplomatic. I was just wanting to call bullshit but was resisting the urge.
I believe MCH may have said it best. "It's your hooch, you get to choose."
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Re: Switching the neutral

Postby shadylane » Mon Jul 09, 2018 3:39 pm

All the router speed controllers I've taken apart, Have the switch on the neutral wire.
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Re: Switching the neutral

Postby NZChris » Mon Jul 09, 2018 5:12 pm

If you switch the neutral, everything before the switch is still live and has the potential to electrocute you, even when it is switched off.
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Re: Switching the neutral

Postby fizzix » Mon Jul 09, 2018 5:13 pm

Here in the U.S., neutral wires are never to have a potential. An open switch on the neutral brings its load side to a potential. Not allowed. Not safe.
The U.S. allows a switched neutral only in conjunction with all poles simultaneously switched.

Otherwise, will it work? Yes. So will two lug nuts on a wheel.

posted with NZChris
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Re: Switching the neutral

Postby Pikey » Tue Jul 10, 2018 3:14 am

Yummyrum wrote:
.................zee255 comments are correct . The same current flows in the live and the Neutral ..................


SO HOW do the Electricity company bill us for KWHours USED ?

And if there is no wattage drop, where DOES the heat come from ?
Last edited by Pikey on Tue Jul 10, 2018 3:27 am, edited 2 times in total.
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Re: Switching the neutral

Postby fizzix » Tue Jul 10, 2018 3:18 am

Current remains the same going in as coming out, but wattage is a measure of the work that current is doing --heat.
You essentially ARE being charged for current at a specific voltage over a given time.
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Re: Switching the neutral

Postby Pikey » Tue Jul 10, 2018 3:22 am

shadylane wrote:All the router speed controllers I've taken apart, Have the switch on the neutral wire.



Thank you :)
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Re: Switching the neutral

Postby Pikey » Tue Jul 10, 2018 3:25 am

fizzix wrote:Current remains the same going in as coming out, but wattage is a measure of the work that current is doing --heat.
You essentially ARE being charged for current at a specific voltage over a given time.


Watts equals Volts x Amps - so if Amps remains equal, are you saying the Voltage drops ?

Something has to change otherwise we cannot be charged for usage
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Re: Switching the neutral

Postby Pikey » Tue Jul 10, 2018 3:29 am

fizzix wrote:Current remains the same going in as coming out, but wattage is a measure of the work that current is doing --heat.
You essentially ARE being charged for current at a specific voltage over a given time.


Congratulations you have invented Perpetual motion !

In fact better than that - you have invented perpetual motion which does WORK without diminishing itself !
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Re: Switching the neutral

Postby fizzix » Tue Jul 10, 2018 3:34 am

I will declare you co-founder in the perpetual motion proceeds!

The amperage draw is a function of the resistance of the heater. Since voltage from the socket remains unchanged, current can only be changed by resistance.
The nature of a resistance is to heat up. It's the simplest conversion of energy there is. But time is an important factor. How long are you using that volts x amps heat?
That's what you're being charged for. Wattage over a given time.
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Re: Switching the neutral

Postby Pikey » Tue Jul 10, 2018 3:48 am

]
zed255 wrote:
In some cases, such as reactive loads ( i.e. capacitive and inductive) like motors and cuircuits containing capacitors, low side or neutral switching can increase the life of a switch by reducing the likelihood of arcing out the contacts. That is an engineering decision.


It sounds as though you are agreeing with what I say, except that you contend that it is tha VOLTAGE which drops taking the associated ampage ( watts = volts x Amps) down with it ? - I can live with that ! 8)

zed255 wrote:In general practise I would encourage switching the live to make a build de-energized when 'off', otherwise the electrical potential is present everywhere in the build only having no path 'out'. Of course we all unplug stuff before working on it, right?


As a matter of safety, if you're working on an appliance with a plug, the plug must always be IN YOUR POCKET !

(The most dangerous appliance to work on is an unplugged one with well wishing people wandering about "Oh here's an appliance and the plug has come out - I'll be helpful and plug it back in !" )

If you're working on a live appliance, ONE HAND should always be In your pocket ! :)
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Re: Switching the neutral

Postby zed255 » Tue Jul 10, 2018 3:49 am

Pikey,

The voltage is effectively 'dropped' by the heating element. You put the supply (live) at one end and it is zero (neutral) at the other. If you think of the element as many small resistors in series and could measure between neutral and any point along the way you would see a progressive drop as you moved from the live to the neutral.

Because a simple heating element is a series circuit, the current is the same throughout. Electric circuits are closed loop.

It is the voltage that provides the electromotive force that 'pushes' the electrons along. When electrons move you have an electric current. When electrons move through a resistance, heat is generated. Think of the heat as being the result of atomic friction, the forced movement of electrons through a medium that resists the movement of electrons.

Seriously though, I respectfully agree with yummyrum, that your level of misunderstanding of electricity makes me cringe a little, no quite a bit really.

Remember, any safety certified device you alter looses its certification. Any device you build from scratch has no certification and must be built with safety in mind (read full and proper fusing and grounding). You will see things done in a double insulated device the DIY'er should NEVER do for themselves. You altered a tea urn. If it electrocutes someone, you are at fault. If it burns your house down, you are at fault. Food for thought. Oh, and maybe a good basic book on electricity.
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Re: Switching the neutral

Postby Pikey » Tue Jul 10, 2018 4:23 am

zed255 wrote:Pikey,

The voltage is effectively 'dropped' by the heating element. You put the supply (live) at one end and it is zero (neutral) at the other........


Thank you for saying that - after I pointed it out !

Pikey wrote:]
zed255 wrote:
In some cases, such as reactive loads ( i.e. capacitive and inductive) like motors and cuircuits containing capacitors, low side or neutral switching can increase the life of a switch by reducing the likelihood of arcing out the contacts. That is an engineering decision.


It sounds as though you are agreeing with what I say, except that you contend that it is tha VOLTAGE which drops taking the associated ampage ( watts = volts x Amps) down with it ? - I can live with that ! 8) ..............


So if you switch the input side you need wiring and switches rated at 240 volts or better. Whereas if you use the same switches and wiring on the neutral side there is much more "Spare capacity". Manufacturers can (And Do) use lower rated wires and switches in these configurations

Much has been made of the potential within the appliance being "Live" at all times. This should not be an issue on an Earthed appliance.

Some, such as microwaves and CRT TVs can still give you a nasty "Bite" AFTER they are unplugged.

For me switching the neutral will be the default way to go in future for heating circuits.

Thanks for helping to refine this thread lads 8)

Hopefully we ALL understand a little better now :)

Sorry @fizzix - perpetual motion machine which does work is now discredited - no "knighthoods" on offer ! :cry:
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Re: Switching the neutral

Postby The Baker » Tue Jul 10, 2018 6:04 am

I just ask my electrician (sigh).

I don't understand it and I don't want to fry.

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