Disconnection Times Again! Fig 3A4

[Mark Wright]

Looking at Fig 3A4 Type B 60898 unless I'm getting it wrong once we In*5 our device there is no way of telling if our disconnection is 10s or 0.1s unless im missing something although this seems quite important??? as our Disconnection time needs to be at least 0.4 seconds and for doing our adiabatic equation for size of CPC.

 

[telectrix]

looking at the graph. all 60898 devices are instantaneous at or above 5xIn

 

[pc1966]

Once you get in to the magnetic trip region the actual disconnection time is probably 10ms or less, but many current-time graphs only go down to 0.1s

If you really need to get the I2t value then typically the manufacturers of the breakers will provide let-through curves for different prospective fault currents.

Safe option is if you are in the magnetic tip region you can assume the minimum plotted, so in this case 0.1s, but as a general rule the let-through energy and thus the adiabatic heating value increases with lower currents as the disconnection time extends.

 

[Mark Wright]

ok but looking at 5*In the line straightens at about 10s slightly above?

10s is not quite in instantaneous is it?

but then our Max Zs values are 5*In and 0.4 seconds is assumed?

 

[telectrix]

the line plummets straight down @ 5 x In.

 

[pc1966]

The 0.4s is the maximum disconnection time for certain systems. For something like a fuse that has a continuous curve you can find a nominal current for every time value, but for a breaker you usually have two regions:

• Thermal inverse-time curve
• Magnetic "instantaneous"

So for most breakers you will find the 5s and 0.4s (typical values for installations for sub-main or final circuit) have an identical current value - the magnetic trip case. It is only when you get something like a 'D' curve MCB that you see different values. From memory:

• B = 3-5 In for magnetic trip
• C = 5-10 In
• D = 10-20 In

 

[Mark Wright]

the line plummets straight down @ 5 x In.

Ok so once the line just goes straight down even though it begins at 10s we can assume this instantaneous? generally 0.4s?

The 0.4s is the maximum disconnection time for certain systems. For something like a fuse that has a continuous curve you can find a nominal current for every time value, but for a breaker you usually have two regions:

• Thermal inverse-time curve
• Magnetic "instantaneous"

So for most breakers you will find the 5s and 0.4s (typical values for installations for sub-main or final circuit) have an identical current value - the magnetic trip case. It is only when you get something like a 'D' curve MCB that you see different values. From memory:

• B = 3-5 In for magnetic trip
• C = 5-10 In
• D = 10-20 In

could I then safely say a breaker under Short-circuit/magnetic In*5 disconnection is 0.4 seconds?

 

[Ian1981]

Ok so once the line just goes straight down even though it begins at 10s we can assume this instantaneous? generally 0.4s?



could I then safely say a breaker under Short-circuit/magnetic In*5 disconnection is 0.4 seconds?

It takes a minimum current of 160 amps to disconnect a 32 amp cb B type as shown by the curve in appendix 3.
What do you think will happen if you have a higher current, say in excess of 160 amps?
In most cases a high current will disconnect a cb in less time than the 0.1 seconds minimum time given in appendix 3, in which case the I2t (energy let through from the circuit breaker )value from the manufacturer will be needed, and in relation to the size of the conductors it protects then the I2t value must be equal to or less then the K2S2 energy with stand value of the conductors.
Remember also that class 3 circuit breakers are current limiting devices, that is they only allow a certain fault level through them.

 

[Mark Wright]

I quite often see the adiabatic equation for size of CPC, Time is assumed at 0.1 but without manufacturers data it seems assuming anything less than 0.4 is not a good idea?

also the adiabatic equation assuming we are within our max Zs of course why not take I as In*5 and t as 0.4?

 

[Ian1981]

I quite often see the adiabatic equation for size of CPC, Time is assumed at 0.1 but without manufacturers data it seems assuming anything less than 0.4 is not a good idea?

also the adiabatic equation assuming we are within our max Zs of course why not take I as In*5 and t as 0.4?

If you do use 0.4, then you’ll find you’ll get a pessimistically high CSA needed for the cpc when in fact it needn’t be that high.
A circuit breaker that achieves its instantaneous tripping current will always disconnect in less than 0.4 seconds and in fact less than 0.1 seconds.
I only find the adiabatic relevant for non instantaneous tripping time’s using fuses as the protective device for example

 

[Mark Wright]

Thanks for the Replys

 

[Mark Wright]

Probably going to get slated

Section 3.5.2 of the On-Site Guide (page 33) TN

A disconnection time of not more than 0.4s is required for final circuits:
• 63 A with one or more socket-outlets
• 32 A when supplying only fixed equipment

A disconnection time of not more than 5 s is permitted for:
• Final circuits exceeding 32 A, and
• Distribution circuits

Final Circuits up to 63A = 0.4 seconds is that correct? but then
Final circuits exceeding 32 A = 5 seconds

what am I missing here?
[automerge]1594500318[/automerge]
this is a mistake I think because it use to be 0.4 for final circuits up to 32A

 

[Ian1981]

Probably going to get slated

Section 3.5.2 of the On-Site Guide (page 33) TN

A disconnection time of not more than 0.4s is required for final circuits:
• 63 A with one or more socket-outlets
• 32 A when supplying only fixed equipment

A disconnection time of not more than 5 s is permitted for:
• Final circuits exceeding 32 A, and
• Distribution circuits

Final Circuits up to 63A = 0.4 seconds is that correct? but then
Final circuits exceeding 32 A = 5 seconds

what am I missing here?
[automerge]1594500318[/automerge]
this is a mistake I think because it use to be 0.4 for final circuits up to 32A

0.4 for socket outlets upto 63 amps.
0.4 for final circuits upto 32 amps.
5 seconds for distribution circuits and circuits above 32 amps.

 

[pc1966]

this is a mistake I think because it use to be 0.4 for final circuits up to 32A

I think they moved socket-finals to treat 63A as for previous 32A

Probably the underlying logic is fixed circuits are less likely to have a conductor fault (no movement) and less likely to be in contact with someone / other objects. But that is simply my speculation.

 

[Mark Wright]

0.4 for socket outlets upto 63 amps.
0.4 for final circuits upto 32 amps.
5 seconds for distribution circuits and circuits above 32 amps.

Thank you Sir! I was happy with your reply but... then Fixed Equipment isn't that 0.4 for fixed 32A equipment and the 0.4 for 63A final circuits of any kind?

it does say 32 A when supplying only fixed equipment - this includes a Ring Circuit?

so still confused???
[automerge]1594502748[/automerge]
The Old onsite Guide says 17th
3.5.2 Disconnection times - TN circuits Table 41.1
A disconnection time of not more than 0.4 s is required for final circuits with a rating (ln)
not exceeding 32 A.
A disconnection time of not more than 5 s is required for
• final circuits exceeding 32 A, and
• distribution circuits.

So it seems like it should have been changed to
final circuits exceeding 63A and Fixed loads above 32A and
distribution circuits can be 5 seconds ???

 

[Ian1981]

Thank you Sir! I was happy with your reply but... then Fixed Equipment isn't that 0.4 for fixed 32A equipment and the 0.4 for 63A final circuits of any kind?

it does say 32 A when supplying only fixed equipment - this includes a Ring Circuit?

so still confused???
[automerge]1594502748[/automerge]
The Old onsite Guide says 17th
3.5.2 Disconnection times - TN circuits Table 41.1
A disconnection time of not more than 0.4 s is required for final circuits with a rating (ln)
not exceeding 32 A.
A disconnection time of not more than 5 s is required for
• final circuits exceeding 32 A, and
• distribution circuits.

So it seems like it should have been changed to
final circuits exceeding 63A and Fixed loads above 32A and
distribution circuits can be 5 seconds ???

Yes it changed with the 18th edition for sockets upto 32 amps require additional protection by an rcd in accordance with 415.1.1 and sockets upto 63 amps have a 0.4 disconnection time.

 

[Mark Wright]

0.4 for socket outlets upto 63 amps.
0.4 for final circuits upto 32 amps.
5 seconds for distribution circuits and circuits above 32 amps.

Sorry Sir I see what you mean a 63A Socket like this ?

 

[Ian1981]

A disconnection time of not more than 5 s is permitted for:
• Final circuits exceeding 32 A, and
• Distribution circuits Final Circuits up to 63A = 0.4 seconds is that correct? but then Final circuits exceeding 32 A = 5 seconds

Sorry Sir I see what you mean a 63A Socket like this ?
View attachment 59410

Yes, The maximum disconnection times stated in Table 41.1 now apply for final circuits up to 63 A with one or more socket outlets and 32 A for final circuits supplying only fixed connected current-using equipment (Regulation 411.3.2.2).
Anything else is 5 seconds (distribution circuits , etc).

 

[Mark Wright]

and Rings etc there all 0.4 s?

so
1 or 2 Socket Outlets like this

up to 63A
32A fixed equipment
and any final circuit like a Ring or Radial upto 32A (but they also need RCD protection)

there all 0.4 seconds?

then anything over these is 5 seconds.

is that right?

 

[Ian1981]

If a circuit has any type of socket outlets on it upto 63 amps then yes it’s a 0.4 disconnection time.
Socket outlets upto 32 amps also require additional protection by an rcd.
However rcd protection can be omitted in non domestic installations by a documented risk assessment.