Adiabatic equation, disconnection times?

[richardindorset]

Just looking to find out where to select 't' the disconnection time for the Adiabatic formula. (This is for college not an actual install.)

Using an EN60898 breaker, 16A. Type B.

Do I look up a datasheet for the breaker?
Do I use table 41.1 in the regs book? Part of regulation 411.3.2.2 (0.4 seconds for a TN system, 230V AC)
Do I assume 0.1 seconds given the range 0.1 to 5 seconds in Fig 3A4 in the regs book?

Please explain which to use. The regs book just says 't' is the operating time of the protective device with 'I' amps fault current. It doesn't say typical, minimum or maximum.

I'd like an answer suitable for C&G exams rather than "I always use blah blah", ideally refering to something in the regs book!

Cheers!

EDIT: I think that there is no specific regulation I can refer to. With a fault current in excess of 5x the rating, the breaker operates in a few milliseconds, so using the 0.1s figure is still very conservative.

 

[JohnW2]

Just looking to find out where to select 't' the disconnection time for the Adiabatic formula. (This is for college not an actual install.)
Using an EN60898 breaker, 16A. Type B.

OK

Do I look up a datasheet for the breaker?

Yes

Do I use table 41.1 in the regs book? Part of regulation 411.3.2.2 (0.4 seconds for a TN system, 230V AC)
Do I assume 0.1 seconds given the range 0.1 to 5 seconds in Fig 3A4 in the regs book?

If you did either of those things you would probably get ridiculous answers, since an MCB will disconnect in milliseconds, far less that 0.4 or 0,1 secs.

Kind Regards, John

 

[richardindorset]

OK

Yes

If you did either of those things you would probably get ridiculous answers, since an MCB will disconnect in milliseconds, far less that 0.4 or 0,1 secs.

Kind Regards, John

Had a look at a couple of datasheets for RCBO's and they only say Type B curve. No typical/min/max disconnection time for the magnetic part of the breaker is specified.

 

[JohnW2]

Had a look at a couple of datasheets for RCBO's and they only say Type B curve. No typical/min/max disconnection time for the magnetic part of the breaker is specified.

You probably need to look in the documentation for graphs (or tables) of I²t for various fault currents, such as this one (rather confusing!) for Wylex MCBs ...

Kind Regards, John

 

[flameport]

where to select 't' the disconnection time

You don't.
You get the I²t value for the device you are using.

In most circumstances, I²t is a single value, representing the amount of energy that can pass when a fault occurs.
Do not consider it as separate things, or what the units of it may or may not be.

 

[JohnW2]

You don't. You get the I²t value for the device you are using.

Quite so.

In most circumstances, I²t is a single value, representing the amount of energy that can pass when a fault occurs.

Although many people seem to call I²t "let-through energy", as I'm sure you understand, it is not a measurement of (or have the dimensions of) any sort of energy - which would be I²Rt. Some just call it "let-through", which is probably less confusing (less wrong!)_.

 

[plugwash]

it is not a measurement of (or have the dimensions of) any sort of energy - which would be I²Rt.

Indeed, assuming the current is constant during the fault, the energy delivered to any given item in the fault path is I²Rt

but r varies between different items in the fault path. Also in the case of cables we care more about the energy per unit length than the actual energy. If a cable is the same size, but twice as long it takes twice as much energy to destroy and has twice the resistance.

For fuses this all works out nicely, the goal is to ensure the fuse element is destroyed before the cable is destroyed and both are energy-driven phenomena. So the fuse maker can specify a single "i²t" value which will be maintained over a wide range of fault currents.

For MCBs, it's less nice, it takes time for the breaker contacts to open. So the let energy tends to increase with the fault current.