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When should a switch be in the Live or Earth line?


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Being no great shakes at electrickery, and challenging myself by starting to rewire my 110 CSW, what criteria are used to locate a switch in the live supply to an accessory, or in its earth line?  All responses will be gratefully received and studied.  Many thanks in advance.

Mike

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Do you want the item to fail on or off? Fail on, then switch the earth side.  Fail off, then switch the live side.

Are you switching at multiple locations or remotely?  Then switching earth is better as it requires less wiring.

Is the device being switched prone to shorting to earth, say from getting submerged?  Then switching live i preferred.

Are you switching electronically?  Switching earth is preferred.

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Some very good points!

Could you explain why switching earth is preferred for electronic switches?

I usually switch live, to avoid the risk of an accidental earth. But I did notice a lot of the standard circuits switch the earth. Even if it's with a simple switch. So following this topic with much interest.

Filip

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2 hours ago, Escape said:

Could you explain why switching earth is preferred for electronic switches?

Semiconductor switches (transistors, FETs, etc.) are easier to fabricate that way round so they tend to switch the ground. It's not *that* rare to switch the supply though, there's enough transistors or smart switch IC's that will do the "high side" out there.

If you're got a physical switch/relay, just switch the live.

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Switching the earth side can make disconnecting a live circuit easier. For example the door pillar switch for the interior lamps is an earth-side switch. If you want to replace or service it, then disconnecting and accidentally touching the wire connecting spade to the metal won't blow a fuse. It just makes the circuit again and the voltage is dropped at the lamps rather than creating a spark of 'uncontrolled' 12v which blows a fuse.

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Someone told me modern vehicles changed over to earth side switching as the switches are less stressed because the voltage has already been mostly expended by the appliance.  I’m not sure that’s true, given that the charge is actually coming from the negative or earth side, the flow actually being the reverse of what is denoted on the circuit diagram.

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1 minute ago, Snagger said:

Someone told me modern vehicles changed over to earth side switching as the switches are less stressed because the voltage has already been mostly expended by the appliance.  I’m not sure that’s true, given that the charge is actually coming from the negative or earth side, the flow actually being the reverse of what is denoted on the circuit diagram.

No, that's a bunch of .... well you know what.

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Having just fualted my LPG system all the injector/solenoids are earth switched by the ECU for pulsing and Power switched for On/Off.

This makes sense as it means that the ECU only needs +5v or similar plus a strong Ground. This way if there is a melt down there is no high current/voltage inside and it is easy to make a strong ground to a component.

 

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3 hours ago, Snagger said:

Someone told me modern vehicles changed over to earth side switching as the switches are less stressed because the voltage has already been mostly expended by the appliance.  I’m not sure that’s true, given that the charge is actually coming from the negative or earth side, the flow actually being the reverse of what is denoted on the circuit diagram.

The same current flows either way it is fitted (and it's really irrelevant as to which way the electrons move). The volt drop is still across the load, not the switch (which has zero resistance). At the moment before switching, there's a 12 volt drop across the switch whatever way it is connected as no current is yet flowing.

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A switch should have minimal resistance, but a dirty switch or underrated one doesn’t - that’s why the light switches on Series vehicles get so hot with halogen conversions.  But it did seem an odd idea that changing the position of the switch would make much difference, hence my scepticism.  I curious what the reasoning behind most manufacturers seeming to have moved from putting the switches in the positive feed to the earth line is - there must be a good reason if most vehicles now have that logic.

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🤣 I knew as soon as I wrote 'zero resistance' that someone would pick up on it. Switching the ground side does reduce wiring assuming that the ground connection is local to the switch.

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3 minutes ago, Peaklander said:

🤣 I knew as soon as I wrote 'zero resistance' that someone would pick up on it. Switching the ground side does reduce wiring assuming that the ground connection is local to the switch.

Given that a lot of stuff earths to the body, that doesn't really save wiring - you still have to run the positive (fused) supply whether there's a switch in that side of it or not, running an earth wire all the way back to the switch rather than earthing to the body nearby would nearly double the amount of wiring.

I suspect a lot of switches now don't connect to the thing they're switching but go via an ECU, possibly via a relay or over CANbus to the actual thing being switched, and the fact the switch just grounds a wire to tell the ECU what's going on is, again, one of those things that's easy to do with solid-state / ECU.

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Here's an example. I wanted to control a spotlight relay. The +ve was easily available at the relay in the engine bay as it was next to the dip & high beam ones. I just ran a single switch wire from the switch at the right hand side of the dash, with the ground very close to it. Otherwise I'd need a feed wire too. I suppose it just depends whether or not the +ve is as accessible as a ground would be. I think your ECU point is a good bet.

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Thank you, everybody, for your responses.  Since Canbus and ECU are foreign words to my 1991 Defender, it seems that I can put the switches in the most convenient places, in most instances.

Mike

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On 9/4/2020 at 6:37 AM, miketomcat said:

I've always switched the live. I was quite surprised when project Binky put there battery cut off on the neg side, don't get me wrong it works but I would of switched the live.

Mike

When the switch is very close to the battery I don't think it's going to make a difference. I now disconnect negative first if I'm removing a battery, for some simple reason, if I hit the spanner against the bodywork / chassis when undoing the positive then it doesn't spark. Even more apt in a Defender battery tray where there's a tendency to hit the seat box with the spanner.

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On 9/4/2020 at 6:37 AM, miketomcat said:

I've always switched the live. I was quite surprised when project Binky put there battery cut off on the neg side, don't get me wrong it works but I would of switched the live.

I guess this follows the logic of undoing the negative side battery terminal first to eliminate the possibility of sparking if the spanner touches any bodywork while undoing the positive side. This isn't really necessary if isolation is achieved by a switch, but it can make the wiring simpler and reduce the chances of a high current short to ground in some circumstances.

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15 minutes ago, nickwilliams said:

I guess this follows the logic of undoing the negative side battery terminal first to eliminate the possibility of sparking if the spanner touches any bodywork while undoing the positive side. This isn't really necessary if isolation is achieved by a switch, but it can make the wiring simpler and reduce the chances of a high current short to ground in some circumstances.

Yes you can put a battery cut off on the neg side but that will only work wheen the engine is off, unlike the FIA switch which will kill the enging when it's running

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39 minutes ago, mmgemini said:

Yes you can put a battery cut off on the neg side but that will only work wheen the engine is off, unlike the FIA switch which will kill the enging when it's running

Yes, you are correct in the sense that if you just isolate the battery (on either side) once the engine is running then the alternator will supply current to keep the engine running until it's shut off by some other means. The FIA switch works by having an auxiliary contact which can be used to shut off the engine so in theory at least you could wire the main contacts into the battery negative and still use the auxiliary contacts to shut off the engine. Whether that's allowed by FIA rules or not, I have no idea. 

The other point which is probably worth including here for the benefit of those for whom electrics are a bit of a mystery is that the battery has an important role in regulating the system voltage so if you do disconnect it without also stopping the engine, you run a fairly high risk of the voltage rising to a level which could cook any sensitive electronics in the vehicle. 

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5 hours ago, mmgemini said:

Yes you can put a battery cut off on the neg side but that will only work wheen the engine is off, unlike the FIA switch which will kill the enging when it's running

What?  A negative side switch kills everything.  There is no difference in function. Most heavy equipment use isolation switches on the negative wire.

Anyone that thinks a positive side disconnect is better/different than a negative side disconnect does not understand how an electrical circuit works. 

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2 hours ago, Red90 said:

What?  A negative side switch kills everything.  There is no difference in function. Most heavy equipment use isolation switches on the negative wire.

Anyone that thinks a positive side disconnect is better/different than a negative side disconnect does not understand how an electrical circuit works. 

So you are quite happy to be able to run rthe engine with the neg wiring switched off to blow the alternator.

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2 hours ago, Bowie69 said:

But the alternator has its own positive and negative....

So, the FIA switch disconnects the alternator as well.

YES. It's nice to know you can switch the engine off via the FIA switch. That's why competition cars have the switch both on the inside and another means of switching the engine off from the outside. Look for the white triangle with the electrical by the lever for the switch

 

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