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If the live wire gets connected to the bare metal body of an electronic device, without any earthing, will the fuse blow up?? My teacher had said that the metal body of the device will then start drawing more current, so the fuse will blow up. But how can any metal surface, whatever the size draw current if the potential difference of the live wire and the body is the same? Please answer, if possible, with links to some established website... I'm gonna have a hard time making my teacher understand.

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Fuses are not designed to prevent electric shock. They are designed to prevent fires. In the time an over-current takes to heat and blow the fuse, you may be long passed your lethal shock.

GFI's on the other hand compare in and out going current, and trip the circuit if there is any difference all on a timescale much faster than your heart.

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Imagine a kettle with a metal body standing on plastic feet.
The live wire touches the metal body and very little current flows if the feet are made of a good insulator and there is little surface moisture on the feet.
The metal body is not disconnected from the live wire because the fuse does not blow as the current flowing through it is not large enough.
If you touch the metal body then a small current will probably flow through you as you are a good conductor and there is probably a conducting path to the earth via your shoes.
That small current will have a large effect on you and you will get an electric shock.

You will note that the earth wire is not there to limit the current through the live wire rather it is there to have a current flow through the live wire which is large enough to blow the fuse and so disconnect the live wire from the device.

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  • $\begingroup$ So doesn't the amount of current flowing depend on the metal surface to which the live wire is connected? What if you keep the same insulation under a large air cooler with a lot more current consumption and a big metal surface to draw the current? $\endgroup$ – ANKiT Nov 6 '16 at 11:35
  • $\begingroup$ I have been writing about the "leakage" current which flows down to the ground when the live wire touches a device. If the resistance between the device and the ground is small all is fine because the fuse will blow. In the past a lot of water containers did not have to be earthed because they were connected to the mains water supply vis copper or lead pipes which in effect acted as the earth wire. With the advent of plastic water pipes metal water containers have to be "bonded"; connected to the earth via a copper wire. $\endgroup$ – Farcher Nov 6 '16 at 11:39
  • $\begingroup$ Thanks for the reply. I've got one more question. Our physics book says that the earth wire and the neutral wire are connected together at the supply centre and then both are exposed to another earth wire which acts as a sink. What's the deal if the wires are connected just behind the socket? For all I know they just need a zero potential for current to flow from live to neutral. $\endgroup$ – ANKiT Nov 6 '16 at 12:00
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    $\begingroup$ Have a read of this link. en.wikipedia.org/wiki/Ground_and_neutral $\endgroup$ – Farcher Nov 6 '16 at 12:03
  • $\begingroup$ Thanks again! No more questions, you have satisfied my hunger!! $\endgroup$ – ANKiT Nov 6 '16 at 12:16
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Judging by your question, it appears that there is some misunderstanding between you and your teacher. I am sure your teacher would not suggest that the metal body of an ungrounded device draws some current, however large the surface of that device might be, but it seems that that was your impression.

NOTE: Since we are dealing with the AC current and the device will have some capacitance, some current, strictly speaking will flow, but this current will be on the order of microamps and therefore won't be able to blow a fuse.

Anyway, I think it might be helpful for you to check one more time how the current flows during the normal operation and under fault conditions, when a hot (live) wire makes a contact with a grounded or ungrounded metal housing of the device.

Here is a simplified diagram showing a typical circuit between a service panel, an electrical outlet and a powered device.

enter image description here

And here are simplified diagrams showing the currents in this circuit under normal and fault conditions.

enter image description here

A couple of observations here.

As long the housing is grounded, it remains at the ground (near zero) potential, even if it touches the hot wire and the circuit breaker or the fuse is still making connection. This is because most of the AC voltage will drop on the long wire connecting the device housing with the power source and only little voltage will drop on the short wire connecting the housing with the neutral wire in the service panel. Still, since this current could be huge, it may quickly overheat the wires and cause fire, so we have to rely on the circuit breaker or fuse to stop it quickly.

If the same fault occurs while the housing is not grounded, no additional current will flow to or through the housing, because there is no return path to ground. This of course could change if a person happens to touch a bare metal of the housing and simultaneously touch some grounded object or stand in a pool of water connected, say, to a grounded pipe, thus creating an alternative return path.

Since this return path has a relatively high resistance in comparison with the ground wire, it'll draw a relatively low current, too low to trip a conventional circuit breaker, but potentially high enough to kill a person.

More advanced circuit breakers, such as RCD's (Residual Current Devices) or CFCI (Ground Fault Circuit Interrupter) outlets can detect such low fault currents and break the circuit soon enough to prevent electrical shocks.

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