Understanding Bolted fault and coodination?

[Sierrasparky]

My question is regarding your every day 10k AIC rated residential equipment. ,

This is the scenario- Say you have a bolted fault line to line on a 20 amp 2 pole 240v circuit. Modern Siemens plug in breakers. I have seen recently that in this situation the 20 amp breaker does not trip but the main did.
Branch and Main same brand and 10k
Is this common?

Does the available fault current play part in this?
The service is only about 8' from the utility tranny. I would assume the available fault current could be greater than 10k which our utility CO states is max for Single Fam res.

 

[electrofelon]

My question is regarding your every day 10k AIC rated residential equipment. ,

This is the scenario- Say you have a bolted fault line to line on a 20 amp 2 pole 240v circuit. Modern Siemens plug in breakers. I have seen recently that in this situation the 20 amp breaker does not trip but the main did.
Branch and Main same brand and 10k
Is this common?

Does the available fault current play part in this?
The service is only about 8' from the utility tranny. I would assume the available fault current could be greater than 10k which our utility CO states is max for Single Fam res.

I though the MB in load centers were usually 22k? Or are you talking about the case of a 100 amp that is backfed as the MB?

 

[mgookin]

See if you can get a waveform for the trip curve on each of the breakers. If the main is faster acting, there's your answer.

 

[jim dungar]

My question is regarding your every day 10k AIC rated residential equipment. ,

This is the scenario- Say you have a bolted fault line to line on a 20 amp 2 pole 240v circuit. Modern Siemens plug in breakers. I have seen recently that in this situation the 20 amp breaker does not trip but the main did.
Branch and Main same brand and 10k
Is this common?

Does the available fault current play part in this?
The service is only about 8' from the utility tranny. I would assume the available fault current could be greater than 10k which our utility CO states is max for Single Fam res.

It is quite common that the main and branch both trip during a bolted fault.
Yes it has to do with the amount of current flowing through the fault, which is based on the amount available from the utility and the impedance of the circuit.

I would not be surprised that the two breakers' trip curves overlap at current as low as 1200A (well below what is available from the utility).

 

[NewtonLaw]

TCC Curves

TCC Curves

My question is regarding your every day 10k AIC rated residential equipment. ,

This is the scenario- Say you have a bolted fault line to line on a 20 amp 2 pole 240v circuit. Modern Siemens plug in breakers. I have seen recently that in this situation the 20 amp breaker does not trip but the main did.
Branch and Main same brand and 10k
Is this common?

Does the available fault current play part in this?
The service is only about 8' from the utility tranny. I would assume the available fault current could be greater than 10k which our utility CO states is max for Single Fam res.

You did not give the specifics for the circuit breakers involved so the TCC curves I show below are an example of when circuit breakers of different current values may over lap for a given fault magnitude. Using a 100 amp main and a 20 amp branch, tripping overlap occurs when faults reach a magnitude of about 900 amps and thus it is a race to see who trips first to clear the fault. The second TCC set shows a 225 amp main and 20 branch. Neither set of CB's coordinates for fault currents above about 2000 amps. This is a long way to say, yes for the CB's you are speaking of, it is common for little or no coordination at extremely high fault currents.

Hope this helps,

Newton Law

 

[Sierrasparky]

Its is hard to see that chart even if I save it and blow it up.

Anyway , If the Available Fault current is lower will that affect anything?
Your homeline example is fine as that is probably similar to 10k curves for Murray and Eaton.

 

[jim dungar]

Anyway , If the Available Fault current is lower will that affect anything?

Roughly:
The downstream 20A breaker is probably in its instantaneous region for all fault currents above 200A (about 10x).
The upstream breaker probably enters its 'maybe' trip point around 5x and its instantaneous region around 8x. what is the handle rating of your main?

 

[Ingenieur]

Another perspective
240 vac 20 A ckt
avail i fault at bus assumed >>> end of 20 A ckt
3% V drop 7.2 V at 16 A
so loop R = 7.2 / 16 = 0.45 Ohm
i fault = 240/0.45 = 530 A at end of run
mid run 1060
lugs >> 1060

much depends on where the fault is and fault Z
Typically coordination between main and branch in a residential application not an issue

 

[Electric-Light]

"The service is only about 8' from the utility tranny. "

Well that's your answer. It doesn't help that it was a line-to-line fault on a dedicated circuit without a bunch of poke-in daisy chain outlets to pad up a bit of impedance. What's the length of the branch to point of fault?

The FC was probably well above the overlap region. If you were to connect 10 breakers or 10 fuses in a chain and applied a large fault current, I wouldn't expect all of them to trip.

Majority of faults are line to ground and most commonly something involving 120v outlets.