2
\$\begingroup\$

For example, imagine a device that comes with a 3-prong (grounded) AC plug. What are the risks associated with, for example, plugging this device into a 2-prong extension cord which effectively disables the grounding as it leaves the grounding prong hanging in mid-air?

I'm interested in both safety risks and device risks.

By a safety risk, I mean a risk to people. E.g., without grounding a short-to-frame could leave externally accessible parts of a device hot with 120V AC.

I'm also interested in device risks. E.g., cases of electrical design where a device would not operate, would operate sub-optimally or with a shorter lifespan that if it were grounded.

\$\endgroup\$
  • \$\begingroup\$ I think you mean "disconnecting the grounding". Bypassing it suggests that you are going to wire around it. You may wish to edit your question title and content to clarify this. \$\endgroup\$ – Transistor Jun 6 '16 at 18:59
  • \$\begingroup\$ Fair enough, although I think disconnecting is also a bit off since it implies the ground was connected, then disconnected, when in fact I'm interested in the effects of never connecting it at all. \$\endgroup\$ – BeeOnRope Jun 6 '16 at 19:08
  • \$\begingroup\$ That's fine. Your question is clear now. See my answer and the link which goes into more depth. \$\endgroup\$ – Transistor Jun 6 '16 at 19:11
3
\$\begingroup\$

The primary purpose of the green-wire ground is USER SAFETY. It provides a SAFE conductive surface for the user to touch if something goes wrong inside the device and allows the live/hot node to become connected to the outside surface.

And then it provides a FAULT PATH back to ground so that the fault current will cause the fuse or circuit-breaker or Residual Current Device or Ground-Fault Interrupter to disable the live power connection.

There should be NO difference in the operation of the device, however. No device should be depending on the green-wire safety ground for ANY operational function. All mains power should be used between the hot and neutral nodes. The Ground node is there only as a safety measure, not for any operational purpose.

Note that there are "double-insulated" gadgets which come with only 2-prong mains power plugs. The only metal parts exposed to the user have been DOUBLE INSULATED from any potential contact with the "hot" mains voltage.

Even without the green-wire safety ground connection, a modern Residual Current Device or Ground-Fault Interrupter will sense that current is going somewhere else (besides returning through the Neutral wire), and it will disable the Live power. However most circuits are NOT protected by RCD/GFI, so it is NOT safe to assume that you can do without any kind of provided green-wire ground connection.

\$\endgroup\$
  • \$\begingroup\$ Thanks! I'm curious why some enclosed devices with no obvious exposed conducting surfaces even include a grounding plug. This is actually a companion question to this one where I was trying to figure out why exactly some device is grounded when it is not apparently necessary. \$\endgroup\$ – BeeOnRope Jun 6 '16 at 19:25
  • 1
    \$\begingroup\$ That is a somewhat different question, and I would refer you to my comment over there. There may actually be no good answer to your question. \$\endgroup\$ – Richard Crowley Jun 6 '16 at 19:30
  • \$\begingroup\$ Indeed, my hope was to solicit your help on that one too. \$\endgroup\$ – BeeOnRope Jun 6 '16 at 20:00
2
\$\begingroup\$

What are the risks associated with, for e.g., plugging this device into a 2-prong extension cord which effectively bypasses the grounding plug?

Firstly, you are not bypassing the ground connection - you are disconnecting it. Bypassing suggests wiring around something, such as "bypassing a fuse with a piece of wire".

Next, consider why we ground appliances in the first place. It is to prevent metallic or conducting cases or enclosures having dangerous voltage levels on them should an internal live wire touch the case. With adequate grounding a large fault current will flow, the fuse will blow or breaker trip and the device will be disconnected from mains.

Without the ground connection the device will remain live and pose a potentially lethal shock hazard to anyone who touches it.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Illustration from How does ground mains work? where this topic is explained.

... imagine a device that comes with a 3-prong (grounded) AC plug.

I'm imagining. In general, if a ground is not required a two-core flex would be supplied and, depending on country, a 2-pin plug fitted. (UK, for example, requires 3-pin connectors on all devices as the earth pin is required to open the socket shutters.)

I'm also interested in device risks. E.g., cases of electrical design where a device would not operate, would operate sub-optimally ...

Some devices may require grounding for noise immunity. For example, audio, TV or computer grounds and internal shields. If the design calls for it then leaving it out may result in interference. It is possible to design isolated but shielded systems - portable devices such as phones.

... or with a shorter lifespan that if it were grounded.

I can't think of any.

\$\endgroup\$
  • \$\begingroup\$ I have edited the question to clarify that I'm talking about simply not connecting the grounding. \$\endgroup\$ – BeeOnRope Jun 6 '16 at 19:10
  • \$\begingroup\$ Thanks for the explanation, but it happened to be the one example I mentioned right in my question. I think that one is well-known - but what are the other risks? Does this risk even apply for a completely contained, plastic component with no exposed conducting elements? \$\endgroup\$ – BeeOnRope Jun 6 '16 at 19:11
  • \$\begingroup\$ See the update. \$\endgroup\$ – Transistor Jun 6 '16 at 19:19
  • \$\begingroup\$ Good points about the shielding functionality of connected grounds. I guess in some cases this can also backfire (e.g., by increasing the chance of ground loops for devices connected into different circuits of the same panel). \$\endgroup\$ – BeeOnRope Jun 6 '16 at 19:24
  • 1
    \$\begingroup\$ Yes, I was looking at a bass guitar amplifier yesterday with a ground-lift button for this very reason. I imagine that it disconnects the audio ground from the chassis ground (which remains connected to mains ground/earth) so that the chassis can't go live. \$\endgroup\$ – Transistor Jun 6 '16 at 19:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.