How important is a grounded stove outlet?

Question

Apologies if covered elsewhere... My place has the old-style three-prong 240V stove outlet, two phases plus neutral. This means the stove (recent, induction) doesn't have a safety ground. The closest ground ever comes to it is if I plug in a grounded appliance on the counter next to it.

The wiring coming to the current outlet is in conduit, so theoretically just replacing the outlet with one that has a ground lug ought to do the job. I just haven't wanted to mess with it; out of sight, out of mind, out of cycles...

I know code says I should fix it. Code is probably right. But must I is gotta do so, or am I probably OK with the grandfathered outlet? I trust this stove more than the one it replaced...

Answer 1

It is an important safety issue. I would do the following:

• Double-check the power requirements for the stove. Really. A lot of induction stoves use a lot of power (that's how they can work so well) so you want to make sure they have the right size wires. For example, if the old stove had a 30A breaker and used a peak of ~ 24A and the new stove is rated for a 40A breaker with a peak of 32A then it is quite possible that you could be pushing the wires to their limits without ever getting a breaker trip. Check both the breaker and the wires.
• If you have metal conduit then check to make sure it is grounded. The easiest way to do that is to check at the receptacle, with the stove unplugged, whether there is continuity between neutral and the conduit. If there is continuity then you have ground and can ground the new 4-wire receptacle or hardwire ground to the metal box. If there is no continuity then there is no ground (even if you have metal conduit - could be a transition somewhere to plastic conduit) and you need to run a new ground wire (green or bare, 10 AWG if 30A and the other wires are 10 AWG, 8 AWG if > 30A or the other wires larger than 10 AWG).

Then you have two choices:

• Replace the 3-wire receptacle with a 4-wire receptacle. Replace the 3-wire cord/plug from the stove with a 4-wire cord/plug.
• Replace the 3-wire receptacle with a blank plate. Replace the 3-wire cord/plug with an appropriate 4-wire wire whip, including appropriate clamps as needed to both the stove and the junction box.

And finally:

• Remove the neutral/ground bond from the stove.

Is this a huge danger. No. But I'd actually posit that while the risk is extremely low initially (large appliances generally don't t have problems for a few years after installation), the risk over time may actually be higher than with a traditional stove. Why? Because with a traditional stove (resistance elements, simple electromechanical controls) the main problems are insulation wearing out or burning up and switch and relay contacts burning up, with induction stoves there are some high-voltage electronics involved that can potentially fail in spectacular fashion. Likely? No. But dangerous if it happens, and that's where the safety protection of a separate ground can be critical.