Grounding Electrical Devices – Grounding Conductor for Device Protection

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I had a home inspection performed on an old house recently. The inspector noticed that some of the outlets had an unconnected third prong. He told me that the absence of a grounding conductor on the outlet wouldn't cause any harm to the home's electrical system itself, but that modern appliances that are designed to work with a 3-prong outlet could be damaged if the grounding conductor is unconnected. He used an example of a television set and mentioned that the t.v. could be fried by an unexpected voltage transient from the grid, and then mentioned something about currents travelling in the wrong direction.

I didn't feel like starting an argument so I didn't press him on this issue, but what he was saying sounded completely bogus to me. I thought the grounding conductor was meant to protect the device user from harm, not the device itself? However, he seemed like a pretty competent guy, so I'm wondering if there's actually any truth to his claims?

Best Answer

There are 2 scenarios where 3 pronged earth-bonded plugs are used.

  • when SMPS are a classic line filter is used to suppress outgoing noise. The noise currents are shunted to earth ground using Y caps on each line to <0.5mA of line frequency max. For safety reasons in case of ground fault and any user touching the metal ungrounded frame and another metal object like a metal plumbing or cable TV does not feel more than a tickle of current.

The line filter is however bi-directional filter and also used to attenuate ~ 3KV power line transients from switching grids or lightning to a lower stress level for components. However the TV was a bad example as most in North America now are all 2 pronged plugs. A PC would be a better example. Not having an earth bond to now relies on the Y cap to attenuation the transient applied to the Common Mode choke. This may or may not be effective in protecting the equipment but now connects the metal case to the line thru the Y caps to a make a unsafe lightning voltage transient on the case.

  • The other scenario is old equipment which do not have double insulated cases like metal hand-held saws and drills and again if the insulation breaks down creates a risk to the user during power line transients.

Earth ground bonding is more a human safety protection 1st.
- 2nd an equipment protector by attenuation for most but not all (eg Florida) lightning storms
- 3rd an EMC compatability noise filter for performance , noise on AM radios etc nearby or noise glitch resetting / locking up / crashing PC

The better solution is to ensure earth bond wires are supplied to all 3 prong outlets and use additional line filters with MOV’s for protecting valued equipment in Florida. ( the lightning capital of North America )

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