What caused my circuit breaker to fail and why is there a 50A fuse?

[Ed Aboud]

Hi all!

I have an issue with the circuit breaker in my house and was wondering would anyone be able to shed some light on what's going on.

So last night, I plugged in the hoover and all the electrical devices in my house switched off. I wasn't surprised as I had many devices on at the time so I went out to the boiler room to reset the circuit breaker.

But to my surprise, I noticed that none of the switches had been tripped. After a few minutes I realized that there was also a 50A fuse in the board (I attached a picture). Luckily there was a new 50A fuse in the boiler room, and I replaced the blown fuse. The electricity came back on and all was good.

What I'm wondering is, and excuse my ignorance (its been a while since I've touched circuit theory), why didn't any of the switches trip?

And why did the 50A fuse blow instead?

Why is there a 50A fuse in the first place, I though that was the purpose of the circuit breakers?

Also, I had a discussion with an employee in an electrical store, he didn't really shed too much light on the problem but just suggested replacing the 50A fuse with a 63A fuse. But, to be honest, I don't feel this is entirely necessary as this was a rare incident.Any help would be greatly appreciated!

Ed(I marked the 50A fuse in the picture)

 

[dlgoff]

It could be that the breakers need some time to heat in order to open while the fuse is "fast acting".

Here's some information on the speed subject. http://en.wikipedia.org/wiki/IEC_60269#1._Speed"

 

[russ_watters]

The 50A is the main path by which all of the electricity flows into your home. The main is always smaller than the sum of the other breakers because odds are you'll never load up every circuit in your house enough to trip it...but 50A is pretty small and in your case, you tripped it.

...btw, is that an actual electrical panel? It looks like it is made of wood and drywall...
I've never seen a panel with that configuration.

 

[Ed Aboud]

Thanks dlgoff for the prompt reply. I'm not too sure about this. Would the circuit breakers not already be hot considering I had many devices on at the time?

Thanks
Ed

 

[dlgoff]

Thanks dlgoff for the prompt reply. I'm not too sure about this. Would the circuit breakers not already be hot considering I had many devices on at the time?

Thanks
Ed

The breaker temperature will be dependent on the current load. i.e. more load means more temperature. When it's too hot, it opens.

 

[Ed Aboud]

The 50A is the main path by which all of the electricity flows into your home. The main is always smaller than the sum of the other breakers because odds are you'll never load up every circuit in your house enough to trip it...but 50A is pretty small and in your case, you tripped it.

...btw, is that an actual electrical panel? It looks like it is made of wood and drywall...
I've never seen a panel with that configuration.

Haha yeah it is the electrical panel, but its actually made of metal.

I still don't understand why that fuse blew instead of one of the circuit breakers switching?

 

[Averagesupernova]

Most likely the reason is that each individual breaker was carrying less than its rated current although probably not much less. When all added together they exceed 50 amps. I have had this happen in a house with an older service. (Fuses)

 

[Ed Aboud]

Most likely the reason is that each individual breaker was carrying less than its rated current although probably not much less. When all added together they exceed 50 amps. I have had this happen in a house with an older service. (Fuses)

I was thinking along the lines of this, but I noticed that on each circuit breaker it says 20A and there are around 30 of them. Does that make a difference?

 

[russ_watters]

Yes, that's what we were saying: 20x30=600 and 600 is a lot bigger than 50.

 

[sportynumair]

Yes, that's what we were saying: 20x30=600 and 600 is a lot bigger than 50.

20 x 30 = 600?? is that how the whole current flow to the main should be calculated?

 

[Redbelly98]

20 x 30 = 600?? is that how the whole current flow to the main should be calculated?

No. The point is that the 50A fuse can blow even when the currents through the individual circuit breakers are all below the breakers' trip point of 20A each.

 

[russ_watters]

Also, assuming this is a US residence, the main fuse might be on a 240V (do they make double-pole fuses...?) line while the other breakers are 120V. So the 50A main is really 100A of 120V service.

In any case, the reason you can have a main that is so much smaller than the sum of the individual breakers is because as we said, you never have all of your circuits fully loaded at the same time. This is called electrical load diversity. Calculating the required main size is somewhat complicated, which is why a licensed electrical engineer is required by law to be the one to do it. The calculations are driven by the National Electric Code (NEC).

For an apartment building, accurate estimation of diversity is critical to properly sizing the incoming service and transformers. For residences, it seems like today people are oversizing them. I have a 100A service on a relatively new 1500 sq ft townhouse with all gas heating/cooking. Even if I ripped out the gas furnace and put in an electric one, I'd still have enough amperage to cover it.