Purpose of Ground Prong

What’s the purpose of ground prong on an outlet, then?


Apart from their use in electronics, which we won’t comment on, and for certain fluorescent lights (they won’t turn on without a good ground connection), they’re intended to guard against insulation failures within the device. Generally, the case of the appliance is connected to the ground lead. If there’s an insulation failure that shorts the hot lead to the case, the ground lead conducts the electricity away safely (and possibly trips the circuit breaker in the process). If the case is not grounded and such a short occurs, the case is live — and if you touch it while you’re grounded, you’ll get zapped. Of course, if the circuit is GFCI-protected, it will be a very tiny zap — which is why you can use GFCIs to replace ungrounded outlets (both NEC and CEC).


There are some appliances that should *never* be grounded. In particular, that applies to toasters and anything else with exposed conductors. Consider: if you touch the heating electrode in a toaster, and you’re not grounded, nothing will happen. If you’re slightly grounded, you’ll get a small shock; the resistance will be too high. But if the case were grounded, and you were holding it, you’d be the perfect path to ground…




How do I convert two prong receptacles to three prong?


Older homes frequently have two-prong receptacles instead of the more modern three. These receptacles have no safety ground, and the cabling usually has no ground wire. Neither the NEC or CEC permits installing new 2 prong receptacles anymore.


There are several different approaches to solving this: 1) If the wiring is done through conduit or BX, and the conduit is continuous back to the panel, you can connect the third prong of a new receptacle to the receptacle box. NEC mainly – CEC frowns on this practice. 2) If there is a metallic cold water pipe going nearby, and it’s electrically continuous to the main house ground point, you can run a conductor to it from the third prong. You MUST NOT assume that the pipe is continuous, unless you can visually check the entire length and/or test it. Testing grounds is tricky – see “Testing Grounds” section. 3) Run a ground conductor back to the main panel. 4) Easiest: install a GFCI receptacle. The ground lug should not be connected to anything, but the GFCI protection itself will serve instead. The GFCI will also protect downstream (possibly also two prong outlets). If you do this to protect downstream outlets, the grounds must not be connected together. Since it wouldn’t be connected to a real ground, a wiring fault could energize the cases of 3 prong devices connected to other outlets. Be sure, though, that there aren’t indirect ground plug connections, such as via the sheath on BX cable.


The CEC permits you to replace a two prong receptacle with a three prong if you fill the U ground with a non-conducting goop. Like caulking compound. This is not permitted in the NEC.


The NEC requires that three prong receptacles without ground that are protected by GFCI must be labelled as such.


See the next section about computers on GFCI-protected groundless outlets.


Learn about Surges, Spikes, Zaps, Grounding and Your Electronics here. >>