Catastrophic oven failure and replacement

Question

Last week, there was a catastrophic failure in the range. I'm fairly confident what happened was that the wiring was compromised, and (one of) the hot wires made contact with the inside of the oven, sending 50 amps through the whole inside of the oven. The short circuit lasted only a few seconds, and when it stopped, I didn't ask questions, I just bolted downstairs and threw the breaker (which was still "on"). It's possible the wire making contact broke away somehow. Overall it was terrifying and I'm glad my family's alive and we still have a home.

1. Is the range circuit ungrounded? I'm a little puzzled why this event didn't cause the breaker to shut the circuit off. I live in an old home that has a mixture of old and new wiring. The receptacle for the range plug is a 4 prong (including ground). Wouldn't the electrical short have sent current down the ground and thrown the breaker? Is it possible that the receptacle, even though it's 4 prong, is ungrounded? Our home is a mix of old wiring and new, but the run from the breaker to the range is very short and completely accessible from the unfinished basement. How can I tell if it's ungrounded?

2. Does a new oven require to be insulated with a bat of fiberglass? Our old push-in oven had a bat of fiberglass insulation. The new one does not. Is this a required addition? Or, in general, is it a good idea?

3. Can the range receptacle be mounted sideways? The replacement range fits differently than the old. The receptacle interferes with the oven, so that it can't back up fully. I'd like to revise the receptacle so that it's mounted transverse and the range can back up to the wall. Is there any issue doing this?

Answer 1

Some minor misinformation here. Breakers used in residential are inverse time--the larger the amount of current the faster they trip. Most breakers will hold 3-5x their rated value for around 10 seconds. Even fuses do this. For the inquisitive type guess what the “sand” inside is doing…

What you should know since you mentioned old is that there are some brands out there that are known to be hazards. Federal Pacific StabLock™ is the leader in failure to trip--lots of articles online if interested (the bus is problematic and the breakers can look like new and won’t trip).

Another brand is Zinsco - these have aluminum buses and in some cases the bus arcs heating the breaker enough so it can’t trip. There is usually a fair amount of damage on the underside before failure.

These are the 2 big ones. Yes any brand breaker can fail and conditions can make or break (pun intended) even the best brand.

As Jack said the wire probably burned clear. Just a few strands of wire can make a really big flash but not long enough or solid enough to trip the breaker is the highest probability if other than the 2 brands above. The fact that it did arc shows there was a return path for the voltage to create the arc.

Answer 2

Your wire probably burned in the clear before causing the breaker to trip and did more arcing rather than having a direct fault. The breaker might have seen the arcing/fault as a large load but not enough to trip.

If you've got a four prong outlet chances are it's grounded but it's easy to check with a good volt meter or examining the main panel and outlet wiring.

As far as insulation, that's a design issue. As long as your new oven is UL listed, you'll be OK.

You'll need to post pictures of your existing outlet inside the box before we can reply on how to relocate it. It should be posted in a new question.

Answer 3

Current down the ground wire doesn't necessarily throw the breaker unless the breaker is GFCI. The GFCI has a trip threshold of 0.005 amps. The breaker has a threshold of 250-500 amps for instant trip, and 51-500 amps for inverse-time thermal trip. The range may have been at 75-100 amps, warming up for a thermal trip, when you manually shut the works off.

The mere presence of a 4-wire socket does not mean it is actually grounded. I would check that.

The range receptacle is usually mounted on a junction box. Sometimes the design allows the receptacle to be rotated.

Sometimes the receptacle is a stand-alone setup. It will have a cable coming into it. It can be repositioned as far as the cable allows, without excessive strain or risk of anything sharp cutting into the cable. You should certainly not force-fit anything here. Pushing the limits of a cable's reach can cause damage to the cable or loss of connection, arc fault and fire inside the receptacle.

As far as the fiberglass, you must follow Code here. National Electrical Code 110.3(B) requires you to install the oven according to its labeling and instructions. (implied: reading them). Do what the instructions say, re: any fiberglass.