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Originally Posted by
darkphader 
I'm in an older house and the outlets have no real ground. Some of them have a NEMA 5-15R receptacle but a tester shows an open ground. Is this of real concern to my AV and/or computer gear...
It is a safety and code issue. If there is no ground, you need to install either 2-prong receptacles or GFCI receptacles with the label "no ground present". The ground is intended to provide a low impedance path so that the breaker in the panel will trip on a fault condition.
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...how can I verify there is an effective path to earth ground?
You can hire an electrician to test the grounding electrode resistance with a megger. Electrical code almost everywhere in USA requires max of 25 ohms. Over that and a second electrode is required.
FWIW surges do not magically disappear into ground. You want a low impedance ground to allow surges such as lightning to return by their normal path, which is the ground. Other surges will return via the electrical service.
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Does the open ground on my receptacles nullify the effectiveness of the whole house surge protector?
What you see at the receptacle is irrelevant to the function of a whole-house device. The quality of the ground at the service entrance is what is important.
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Does the open ground nullify the effectiveness of both MOV type and series mode surge protectors at point of use?
Won't affect a single mode surge protective device, either MOV or series mode. With a 3-mode MOV-based device, you will lose 2 of the modes. You will still have L-N protection, which is what the inventor of series mode protection recommends.
Edited by Colm - 3/8/13 at 4:16pm