Smaller breaker didn't trip, why?

[cadunkle]

I had an electrical mishap tonight, shorted line to ground on a 30a 240v circuit in my shed. It tripped a 40a breaker in the house rather than a 30a breaker in the shed.

In more detail, the shed panel is fed from the garage panel with 6ga wire and 50a breaker. The garage panel is fed from the house with 6ga wire on a 40 amp breaker. The 40a breaker in the house tripped, killing all power to the garage and shed rather than just the circuit that shorted to ground. Why might this be? Slower response time on the 30a breaker in the shed? Old 40a breaker in the house tripping at a lower load due to age?

 

[nadogail]

Some brands of breakers are not as sensitive as others. Zinsco were regarded as the worst.

 

[Norcal]

Some brands of breakers are not as sensitive as others. Zinsco were regarded as the worst.

FPE is notorious for failing to trip on a fault. Zinsco not as bad, but that is not saying much.

 

[MBfreak]

Many series connected magnetic/overcurrent breakers do not trip "correctly" since their trip curves are quite "wobbly"
This is more pronounced at a dead short,
In transmission system protection and control circuits it is accepeted paractice that the downstream MCB is a fast design and two amapcity ratings below the MCB feeding it
Ie distribution MCB slow trip 16 A and unit prot MCB fast trip 6A
This goes for European type MCBs from Siemens, ABB etc.
As usual I know NOTHING about US praxis .

Ola

 

[TRWham]

If I read correctly, everything in the shed and garage is on that 40 A breaker, so it already had other loads on it, plus it was the first to see the current.

 

[wyliesdiesels]

Some brands of breakers are not as sensitive as others. Zinsco were regarded as the worst.

Fire Pacific Electric/FPE is worse than shitsco/zinsco...

 

[wyliesdiesels]

I had an electrical mishap tonight, shorted line to ground on a 30a 240v circuit in my shed. It tripped a 40a breaker in the house rather than a 30a breaker in the shed.

In more detail, the shed panel is fed from the garage panel with 6ga wire and 50a breaker. The garage panel is fed from the house with 6ga wire on a 40 amp breaker. The 40a breaker in the house tripped, killing all power to the garage and shed rather than just the circuit that shorted to ground. Why might this be? Slower response time on the 30a breaker in the shed? Old 40a breaker in the house tripping at a lower load due to age?

Brand and model of breakers?!

Without that info no way for us to give u answer as to whether it was age or something else

 

[Bert_]

The amount of current to instant trip a 30 amp versus a 40 amp breaker is probably very close. Add in some variation between each individual breaker and it's no surprise that the 40 could trip first.

Here's an example of a trip curve, you would be looking at the straight part towards the bottom since that's the magnetic trip portion of the breaker.

There are a few brands of breakers out there that didn't have a magnetic trip function. They relied on the thermal overload detection only. They were very slow to trip on short circuits.

 

[alfredeneuman]

There are a few brands of breakers out there that didn't have a magnetic trip function. They relied on the thermal overload detection only. They were very slow to trip on short circuits.

Federal Pacific didn't have mag trips....Neither did Bulldog pushmatics.
Curiously, the "Magnetrip" feature was perhaps Zinsco's only decent decision

 

[jimbee]

There can be a lot of overlap in the response curves of molded case circuit breakers. Coordination of circuit breakers with nominal trip ratings that are close can be almost impossible.

 

[FredWanaker]

If I read correctly, everything in the shed and garage is on that 40 A breaker, so it already had other loads on it, plus it was the first to see the current.

probably this.^^^^

 

[ZX3ST]

Some brands of breakers are not as sensitive as others. Zinsco were regarded as the worst.

The amount of current to instant trip a 30 amp versus a 40 amp breaker is probably very close. Add in some variation between each individual breaker and it's no surprise that the 40 could trip first.

Here's an example of a trip curve, you would be looking at the straight part towards the bottom since that's the magnetic trip portion of the breaker.

There are a few brands of breakers out there that didn't have a magnetic trip function. They relied on the thermal overload detection only. They were very slow to trip on short circuits.

There can be a lot of overlap in the response curves of molded case circuit breakers. Coordination of circuit breakers with nominal trip ratings that are close can be almost impossible.

This pretty much sums it up. Lots of potential variables here.

I have a Square D Homeline 200A main, with a Cutler Hammer 60A sub in my home. (Previous owner did an addition)

ANECDOTE:
I mistakenly miswired (shorted) a baseboard heater thermostat on a 20A branch of the sub panel, which tripped nothing in the subpanel, but instantly tripped the feeder breaker AND the main breaker in the main panel.

 

[wyliesdiesels]

If I read correctly, everything in the shed and garage is on that 40 A breaker, so it already had other loads on it, plus it was the first to see the current.

that would be an overcurrent situation. The OP had a short circuit. Different functions in the breaker....

 

[pattenp]

My question is why is the garage panel fed from house with a 40A breaker and the shed fed from the garage panel with a 50A breaker. I would reverse that.

 

[wyliesdiesels]

i wondered the same thing

 

[cadunkle]

Brand and model of breakers?!

Without that info no way for us to give u answer as to whether it was age or something else

House main panel (60a GE THOL) > house generator transfer switch panel (40a Siemens QP) > garage panel (50a SquareD HOM) > shed panel (30a SquareD HOM) > electric heater.

My question is why is the garage panel fed from house with a 40A breaker and the shed fed from the garage panel with a 50A breaker. I would reverse that.

That's how it was when I got the house. An electrician owned it before me and since new. I get the impression buying an electrician's house is like they say about buying a mechanic's car. Plenty of nice to have stuff, some overly elaborate, and some quick and dirty obviously with take-outs or leftovers from jobs.

The garage was unfinished with a split bus panel when I got the place. I finished the interior of the garage, ran a bunch more outlets, better lighting, and installed a new Homeline panel.

The shed was fed from the garage with two circuits using Romex through one conduit (10/2 for a 30a camper plug, 12/2 for a 20a circuit for lights and outlets in shed). I pulled that out and ran 6/3 NM through the walls to reach the conduit to shed, then 6ga THHN through the existing conduit to a new Homeline panel. 50a breaker since the 6/3 NM is only rated for 55a. I have a 5HP compressor and 5k watt electric heater in the shed. Garage electrical load is fairly small, just lights, computer, gas heater, and whatever I plug in. I need far more amps in the shed than the garage.

The garage panel appears to be fed using 6ga THHN, at least that's what's at the 40a breaker in the house and the main breaker in the garage panel. I'm guessing it's the same with the whole length but haven't yet checked every pull point or junction box along the way. I'll swap it to a 60a breaker if it's entirely 6ga wire. Given the mixed breakers and bits in everything I'm guessing the original owner had a 40a Siemens breaker on hand at the time and used that instead of buying a larger breaker to match the panel.

I haven't done much with the wiring in the house. I'd like to put in a new panel with a whole house automatic transfer switch ahead of it, eventually.

 

[rlitman]

House main panel (60a GE THOL) > house generator transfer switch panel (40a Siemens QP) > garage panel (50a SquareD HOM) > shed panel (30a SquareD HOM) > electric heater...

These are all going to be similar enough in design that it's a race as to which one(s) trip first. Being just a tad slower could mean the upstream trips, and the downstream holds.

The trip curves on the SquareD QO series are a bit faster than Homeline (around 20% off the tip of my head), but there's a lot of variability here that means anything could have happened and can still if you repeat the experiment (don't).

Breaker coordination is difficult even in the best of cases, and residential breakers are CHEAP. I've seen a 20A two phase 208V breaker trip a 30A three phase above it, and a 225A above that, all because the circuit had a shockingly low impedance. That 225A breaker (SquareD JGA36225; why do I know the part number off the top of my head...) has an MSRP of $3272, yet its construction isn't radically different than your Homeline gear. The 1000A breaker upstream of that held, likely because it's 10x more expensive micro-controller design intentionally holds back tripping for a few cycles, assuming a downstream breaker will clear the fault. But this sort of sophisticated engineering isn't going into a residential setting, when a main breaker costs as much as a car. It's commonplace in transmission as MBfreak points out.

 

[TRWham]

that would be an overcurrent situation. The OP had a short circuit. Different functions in the breaker....

It is true that a circuit breaker responds differently to these 2 problems, but it's the same function as far as the breaker is concerned. A circuit breaker only speaks one language: overcurrent. It doesn't know why the current is high, just that it is and how fast it got that way. The 2 factors work together to determine how fast it trips.

 

[dvcochran]

I had an electrical mishap tonight, shorted line to ground on a 30a 240v circuit in my shed. It tripped a 40a breaker in the house rather than a 30a breaker in the shed.

In more detail, the shed panel is fed from the garage panel with 6ga wire and 50a breaker. The garage panel is fed from the house with 6ga wire on a 40 amp breaker. The 40a breaker in the house tripped, killing all power to the garage and shed rather than just the circuit that shorted to ground. Why might this be? Slower response time on the 30a breaker in the shed? Old 40a breaker in the house tripping at a lower load due to age?

You have a 40-amp breaker feeding a 50-amp breaker? Problem #1. Is the 40-amp that tripped feeding anything else?

 

[wyliesdiesels]

That's how it was when I got the house. An electrician owned it before me and since new. I get the impression buying an electrician's house is like they say about buying a mechanic's car. Plenty of nice to have stuff, some overly elaborate, and some quick and dirty obviously with take-outs or leftovers from jobs.

ummm are you sure he was an electrician because....

The shed was fed from the garage with two circuits using Romex through one conduit (10/2 for a 30a camper plug, 12/2 for a 20a circuit for lights and outlets in shed). I pulled that out and ran 6/3 NM through the walls to reach the conduit to shed, then 6ga THHN through the existing conduit to a new Homeline panel. 50a breaker since the 6/3 NM is only rated for 55a. I have a 5HP compressor and 5k watt electric heater in the shed. Garage electrical load is fairly small, just lights, computer, gas heater, and whatever I plug in. I need far more amps in the shed than the garage.

NM-b/ romex is not permitted to be ran underground or outdoors

 

[rlitman]

It is true that a circuit breaker responds differently to these 2 problems, but it's the same function as far as the breaker is concerned. A circuit breaker only speaks one language: overcurrent. It doesn't know why the current is high, just that it is and how fast it got that way. The 2 factors work together to determine how fast it trips.

He's getting at the difference between the thermal and magnetic functions of the breaker. They're not remotely the same functions, even if both are related to overcurrent.

You have a 40-amp breaker feeding a 50-amp breaker? Problem #1. Is the 40-amp that tripped feeding anything else?

Sounds to me like a 40A breaker supplying a panel with a 50A main in it. Nothing wrong with that.

 

[TRWham]

He's getting at the difference between the thermal and magnetic functions of the breaker. They're not remotely the same functions, even if both are related to overcurrent.
...

Fair enough. A CB may trip magnetically on a motor lock up as well, but in this case it's pretty clear that did not happen.