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Whenever I'm working in labs, I have always disconnected the +V terminal first because I've always been taught powering a chip without a ground is dangerous and likely to blow the chip. I've also experienced that happening so I'm pretty sure it's true.

However, the other day I was messing around in my car and long story short the battery died. I was instructed to disconnect the ground first because if I was disconnecting the +V and accidentally shorted it to a piece of metal the circuit would still be complete and a whole bunch of amperes would flow and perhaps electrocute me.

Now it seems to me that even with the ground terminal disconnected, if I short the +V of a battery to say the chassis of a car with something like a screw driver, won't current still flow?

Are there other reasons for the reverse paradigm? Any other safety considerations I should be aware? Thoughts in general on this situation?

pipe
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NickHalden
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2 Answers2

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The main reason for disconnecting the negative terminal on a car battery first is that the whole of the car body is linked to that negative terminal.

Disconnecting it first removes all the possibility of a short between positive and negative without going direct to the battery negative terminal.

For disconnecting individual items within the car, disconnect the positive first as you would with any other device.

As for powering chips without a ground (or \$V_{SS}\$) connection, the power will probably try and find its way out somewhere and yes, could cause problems.

Personally I usually turn off the power supply rather than unplug it live.

Majenko
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  • Hm, makes sense. But then what about all the devices who's grounds are connected to the car chassis which is no longer connected to the ground on the battery? They are all essentially plugged into +V without their ground connected right? – NickHalden Jul 10 '11 at 01:15
  • The grounds may be linked to each other (what we call a ground plane), but with no link to battery negative there is no circuit. Current cannot flow. – Majenko Jul 10 '11 at 08:12
  • So how is that different from an IC when you disconnect ground first? – NickHalden Jul 10 '11 at 20:19
  • An IC has more than one route to ground. For example, if you take a PIC chip, connect it up, with some LEDs linked from say port B to ground, then disconnect the ground pin from the chip, the LEDs will light up dimly as the power flows through the chip and then through them to get to ground. With say a car stereo you have only one route to ground (well, two counting the case, but by the time you're disconnecting the ground you have it out of the chassis). But still, disconnecting a _component_ in a car should be done the same as disconnecting a component in a circuit. – Majenko Jul 10 '11 at 20:27
  • ... Disconnecting the power source in the car (the battery) should be done negative first as the - is connected to the most available point of contact: the car body. With no negative connected to the battery there is absolutely zero chance of causing a short circuit no matter how hard you try unless you actually go to the battery negative terminal and touch something direct to that. Also, a lot of cars have multiple connections to the positive connection on the battery, but only one heavy duty negative connection, so it is a lot easier to reconnect it afterwards. – Majenko Jul 10 '11 at 20:30
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When disconnecting either terminal of the battery, no current can flow! Therefore no harm can result to any component. The choice is determined by whichever terminal is more easily available.

K. Bourque
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