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To break off any circuit, the common way is to disconnect the negative and or ground terminal. I rarely see people disconnecting the positive and if so, they're not well educated in electrical engineering most of the times.

Well, I've no electrical degree either and to me, breaking the circuit off at the negative side seems counter intuitive.

In my intuition, breaking the input makes more sense than breaking the output, due this is the side where the energy is coming from.

Why to disconnect the negative/ground terminal, not the positive?

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    \$\begingroup\$ Might the reason be, to discharge given capacitors or such? \$\endgroup\$ – Sempie Apr 26 '16 at 9:52
  • \$\begingroup\$ Please show an example of this - the example will likely hold the inherent reason for negative disconnection. \$\endgroup\$ – Andy aka Apr 26 '16 at 10:33
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    \$\begingroup\$ @Andyaka, Topical for me and to answer your question: for any non-trivial service Land Rover service guidelines start with disconnecting the negative terminal of the battery before any work should proceed... \$\endgroup\$ – Lamar Latrell Apr 27 '16 at 5:23
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There is no real inherent distinction between breaking one side of a loop or the other side- it's all in series so breaking the negative or the positive side of the supply keeps electrons from flowing.

When you have an electronic switch and are turning off part of the circuitry with other circuitry it's easier to break the negative - called a low side switch (requires fewer parts), but if some other circuitry remains connected, that may cause problems. For example, if I break the ground connection on a module, the 'input' pins may start sourcing current back to the controller (because otherwise they would have to go negative with respect to 'ground' on the module)- it won't necessarily switch the current off completely and it may even damage the controller or module in some situations.

Look at a few of the many answers on this site where people have tried this, failed and a high-side switch has been suggested. It comes up quite regularly.

If it's something completely isolated like a relay coil, most designers will use a low side switch because it's simpler and there is no advantage the other way. In cars, the chassis is used as a return, so high side is preferred if the load is remote. Here is a useful document on automotive applications.

Speaking of automotive, there is one particular situation worthy of mention where removing the negative connection is recommended for safety reasons- and that is when you are working on a car. Since the negative terminal is almost always connected very solidly to the chassis, if you try to remove the positive terminal with a (conductive) wrench/spanner and the tool touches the chassis hundreds of amperes will flow, causing the wrench to get red hot. Some people have left their wedding or other rings on and receive severe burns (to the point of possibly losing their finger) when the ring formed part of the circuit.

So take the negative terminal off first and put it on last if you are working on a car. And remove jewellery.

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    \$\begingroup\$ Perfectly answers my question. \$\endgroup\$ – Sempie Apr 26 '16 at 11:52
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    \$\begingroup\$ @Sempie It is generally not recommended to accept an answer immediately as it discourages others from contributing to the conversation - others will be discouraged from answering since the 'bonus' for acceptance has already been awarded. Waiting 24 hours or so is a good practice. \$\endgroup\$ – Adam Lawrence Apr 26 '16 at 12:01
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    \$\begingroup\$ Agree with Adam - 24 hours is a good rule of thumb - you can unaccept. \$\endgroup\$ – Spehro Pefhany Apr 26 '16 at 12:03
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Most circuits that control switches are ground-referenced. It is therefore easier for them to drive a ground-based switch than a supply-based switch. The former is termed low side switching and the latter high side switching.

Both methods are perfectly valid. Like most things in engineering, it's a tradeoff. No, this is not a indication of the education level of the engineer. Good engineers evaluate lots of options, then choose the one that is the best fit for the particular design.

Low side switching is often chosen because it's usually simpler to achieve. In many cases, it doesn't matter whether the positive or negative power connection is broken. Good examples are driving a solenoid, relay, or LED.

When a circuit is more complex and has connections back to the driving circuit, then it's often worth the extra complexity of switching the power and keeping the ground always connected. For example, if you're switching the power to digital module with I/O lines back to your micro, it may be advantageous to keep both grounds connected permanently and switch the power to the module.

Again, everything is a tradeoff. There is no universal single right answer.

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From the point of view of the circuit that is disconnected, it does not make any difference at all. There is no way that is better than the other.

Now, if is is disconnected/connected using an active device, e.g. a transistor, instead a simple mechanical switch, it starts making a difference. If you put the transistor between the circuit and its ground, you'd use a NPN transistor, which are usually cheaper than the equivalent PNP transistor. Same for NMOS/PMOS FETs. Moreover, in this case, you rarely need additional level shifting.

On the contrary, if you decide to break the circuit from the positive side, you'd use a PNP BJT or PMOS FET, which are more expensive, and it may require an additional level shifting stage if the load circuit is not powered by the same supply as the control circuit.

So, it is usually simpler to choose to break at the ground.

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The reason is pretty simple: If you accidentally touch the conducting components on the PCB board or any other grounded metal part of the PCB with the positive terminal connected, nothing will happen.

But if the negative terminal is still connected & positive terminal disconnected first, and if you happen to touch any metallic parts/Components, it would result in disastrous consequences, since the battery might be conducting several amperes.

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    \$\begingroup\$ Makes sense so far. But then, if I touch the board with my skin, I might become the ground and thus, which seems worse than breaking a board. (Depending on voltage and max current, of course. \$\endgroup\$ – Sempie Apr 26 '16 at 10:07
  • \$\begingroup\$ your skin should not be ground when experimenting with electronics, use a galvanic isolation to power your circuit \$\endgroup\$ – Christian Apr 26 '16 at 10:27
  • \$\begingroup\$ Sure sure, but this was not the question. For example, when when bridging over a car battery, galvanic isolation might barely be possible. \$\endgroup\$ – Sempie Apr 26 '16 at 10:39
  • \$\begingroup\$ @Sempie: Please understand that for current to flow it needed closed loop system. So coming to your doubt you will never have one common ground when you touch.Two different Grounds are 1.)Grounding by Earth surface. 2.) Grounding on PCB Board. Your body provides resistance between this two grounds. I hope you understood this basic concept. \$\endgroup\$ – Viral Embedded Apr 26 '16 at 10:48
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    \$\begingroup\$ I'm struggling to see what your answer is explaining. Please elaborate. You say it's a pretty simple reason but I don't follow your answer. \$\endgroup\$ – Andy aka Apr 26 '16 at 11:02
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The answer is... it depends, ideally you disconnect the terminal which is connected to the exposed metal (which is usually connected to ground/negative), that way any exposed conductors are safe. If the battery only has two leads, it doesn't matter as much because a break anywhere in the loop will prevent current from being able to flow, current needs a return path, as long as there isn't one, it's safe. When working with high voltages (or earthed supplies as you'd have two HV terminals which can pump current through you to earth) I'd disconnect ALL power leads, that way you know there's no way for current to flow.

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  • \$\begingroup\$ No, you don't switch the conductor connected to the exposed metal. You keep the exposed metal grounded and switch the other side of the power supply. Here in the UK at least there are copious regulations about keeping everything Earthed... and the Earthed connection ultimately runs back to the centre tap of the three phase transformer feeding the mains supply. \$\endgroup\$ – Ian Bland Apr 27 '16 at 9:45
  • \$\begingroup\$ Unless the exposed metal is live (like in old crt tv's and some SMPS's), but you're right, keep ground remove live conductors \$\endgroup\$ – Sam Apr 28 '16 at 0:52
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I don't understand the question. All electrical switches are in the live conductor, not the (grounded) neutral conductor. This is for obvious safety reasons; if you disconnect the neutral side, everything remains at live potential which is very dangerous. Some switchgear is double pole (it breaks both conductors, or all four in a three phase system), but single pole switching is always on the side which isn't earthed. Like the fuses and circuit breakers.

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    \$\begingroup\$ I believe it was DC they were discussing, not AC. \$\endgroup\$ – Tim Spriggs Apr 26 '16 at 23:58
  • \$\begingroup\$ Makes no difference. If one side of a supply is grounded, you switch the other one. If neither side is grounded, you double pole switch both. If it's a three rail DC supply you'd either switch the two outers (grounded) or all three poles (isolated). Switching goes on the side that will render the circuit beyond the switch safest in the off position. If you switch the grounded side of a supply, it will not prevent shock risk, nor isolate a fault, with potentially catastrophic consequences. \$\endgroup\$ – Ian Bland Apr 27 '16 at 0:32
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In my intuition, breaking the input makes more sense than breaking the output, due this is the side where the energy is coming from.

This is actually NOT the case. Electron flow actually runs from negative to positive. The convention of +/- was invented before this was understood.

https://van.physics.illinois.edu/qa/listing.php?id=583

Regardless, this does not matter since breaking the circuit at any point stops the "energy flow". Negative is generally used for the ground, as others have mentioned, so disconnecting the negative is the safest approach.

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The battery terminal to be disconnected first depends on the terminal that is earthed or attached to the car's body.When a car is negatively earthed then it is prudent to take off the negative terminal first and connect it last. On the other hand when a car is positively earthed then it convenient taking off the positive terminal first.For instance, when a car is negatively earthed and you disconnect the positive terminal, it means the whole body of the car is conducting,(because the negative terminal is attached to the body)If there should be a mistake by using a metal or any conductor to touch the positive pole of the battery and car's body, then there will be severe damage to your car's sensors or ETACS or set your car on fire.

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Which terminal to disconnect first (the grounded one or the ungrounded one) is primarily a safety concern of the service technician.

The grounded terminal should be disconnected first to ensure that a battery dead-short does not occur, should the spanner disconnecting the other one contact a nearby grounded metal part.

A battery dead-short will result in short circuit currents of the order of hundreds of amperes and heavy sparks. This could result in the service technician suffering burns or other injuries due to involuntary movement.

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protected by W5VO Mar 21 '17 at 18:49

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