Earth currents

By Marcus Wilson 29/11/2010

Five hundred and seventeen for one.  That’s more like it.  Looking forward to more of the same in Adelaide.

So, physics. Last week I was doing a bit of work in the lab with a  student, trying to track down why his instrumentation wasn’t working. We’re still at it; what he’s trying to do is quite complicated, but we made some progress. One thing we noticed was that there were multiple ground points in the circuit.

What do I mean by this?  A lot of electronic equipment is earthed (‘grounded). It’s basically an electrical safety thing. It means that any metal on the outside of the equipment (stuff you can touch) is connected, via the power cable and the earth pin on the plug, to a large piece of metal somewhere outside the building that is hammered well into the ground. What this means is that the outside of the equipment sits at the same electric potential as your feet. If you touch the case with your hand, there is no potential difference between your hand and your feet (which are touching the ground, usually), and so no electric current flows through you. If there’s an electrical fault and the outside case suddenly becomes live, a large current will flow through the earth wire to ground, blowing the fuse.

But a problem can happen if you are using many pieces of electrical equipment as part of an electrical circuit. For example, the negative input to an oscilloscope is often (but not always) connected to ground. When you measure the potential difference in your circuit with such an oscilloscope, the point where you connect the negative terminal becomes ‘grounded’. If you then used a second oscilloscope to monitor another potential difference, you could have two points in your circuit that are forced to a ‘ground’ potential.

That doesn’t work. What happens is that you have introduced a short-circuit between the two ground points. In effect, you’ve connected a wire between them. However, the ‘wire’ is slightly obscure – it journeys from the negative input of the first oscilloscope, down the earth wire of the power cord, where it probably joins somewhere with the earth wire of the power cord from the second oscilloscope, then it goes up that earth wire and to the negative terminal of the second oscilloscope.  There could be a substantial current flowing around this loop. What it will do to your circuit will depend on what the circuit is, but for sure it will mean that the circuit won’t do what you want it to.

Usually, with a spot of thinking, you can reorganize what you’re plugging in to the circuit to measure it so that you avoid the problem.

The moral – you can only have one earth point on a circuit. Watch what you plug in to monitor the circuit – it may be earthed.

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