0.4 seconds or 5 seconds

[David345]

Just a quick question. This might sound like a stupid question so apologises. I'm slightly confused with the regs 18th edition when it comes to disconnection times. So I was taught in college that in the 17th edition that when a circuit is less than 32A then the maximum permitted time is 0.4 seconds and anything over 32A then its 5 seconds. Now in the 18th edition has it has been changed? So that anything under 63A supplying socket outlets its 0.4 seconds and anything over 63A its 5 Seconds. And also why would you have socket outlets on a 63A breaker when it should be on a 32A ring or 20A radial. Sorry if those questions sound stupid, it's just I can't get my head around it for some reason. Thanks.

 

[Flanders]

Just a quick question. This might sound like a stupid question so apologises. I'm slightly confused with the regs 18th edition when it comes to disconnection times. So I was taught in college that in the 17th edition that when a circuit is less than 32A then the maximum permitted time is 0.4 seconds and anything over 32A then its 5 seconds. Now in the 18th edition has it has been changed? So that anything under 63A supplying socket outlets its 0.4 seconds and anything over 63A its 5 Seconds. And also why would you have socket outlets on a 63A breaker when it should be on a 32A ring or 20A radial. Sorry if those questions sound stupid, it's just I can't get my head around it for some reason. Thanks.

You can get 63 amp industrial sockets
[automerge]1598043950[/automerge]
Just to add the disconnect time are the same as the 17th for fixed equipment and distribution circuits it's just for socket outlets that have changed .

 

[David345]

You can get 63 amp industrial sockets
[automerge]1598043950[/automerge]
Just to add the disconnect time are the same as the 17th for fixed equipment and distribution circuits it's just for socket outlets that have changed .

Oh yeah I cabt believe I forgot about those. They are 3 phase sockets. Thanks
[automerge]1598044279[/automerge]

You can get 63 amp industrial sockets
[automerge]1598043950[/automerge]
Just to add the disconnect time are the same as the 17th for fixed equipment and distribution circuits it's just for socket outlets that have changed .

So just to clarify, previously in the 17th edition did it only Include sockets upto 32A but now in the 18th edition it includes sockets upto 63A?

 

[Flanders]

Oh yeah I cabt believe I forgot about those. They are 3 phase sockets. Thanks

Yes you can also get single phase ones aswell
[automerge]1598044361[/automerge]

Oh yeah I cabt believe I forgot about those. They are 3 phase sockets. Thanks
[automerge]1598044279[/automerge]

So just to clarify, previously in the 17th edition did it only Include sockets upto 32A but now in the 18th edition it includes sockets upto 63A?

Correct

 

[David345]

Yes you can also get single phase ones aswell
[automerge]1598044361[/automerge]

Correct

Cheers mate. Thanks for the help, much appreciated.

 

[Dartlec]

It's also different for TT systems, just to complicate matters.

For a TN system it's 0.4 seconds for final circuits up to 32A (without socket outlets) or up to 63A where there are socket outlets (this includes the industrial sockets mentioned by Flanders that can be rated at 63A).

In most domestic situations you won't generally see a socket circuit above 32A as you say.

On a TN system, for any other distribution circuit or circuit not covered by the first rules 5 seconds is permitted.

For a TT system, the times are 0.2 seconds and 1 second respectively.

This is all assuming 230V AC - they are different again for higher voltages.

 

[David345]

It's also different for TT systems, just to complicate matters.

For a TN system it's 0.4 seconds for final circuits up to 32A (without socket outlets) or up to 63A where there are socket outlets (this includes the industrial sockets mentioned by Flanders that can be rated at 63A).

In most domestic situations you won't generally see a socket circuit above 32A as you say.

On a TN system, for any other distribution circuit or circuit not covered by the first rules 5 seconds is permitted.

For a TT system, the times are 0.2 seconds and 1 second respectively.

This is all assuming 230V AC - they are different again for higher voltages.

Right I'm starting to understand more now. Thanks for the help.

 

[Dartlec]

Right I'm starting to understand more now. Thanks for the help.

Some of is slightly moot other than correctly filling in certification, since RCD protection is now almost across the board in new installs so disconnection times are easily met.

It does make sense that anything with a socket outlet should be 'safer' since at some point a human will be plugging or unplugging things.

Maybe the change had something to do with car charging points becoming more common, (though I think they're 32A mostly at the moment) or to cover any possible use of higher power sockets to come, but that's purely guesswork..

 

[pc1966]

The most common case (TN) for the 5s disconnection is on a sub-main, there are two reasons for having a longer ADS time:

• Potentially better selectivity with downstream OCPD (generally with a fused sub-main feeding a sub-board with MCB) where you would design for as high a fuse rating as safe (ADS time & adiabatic limits)
• On high current circuits the supply Ze might not allow a faster time in any practical choice of cable, etc.

Generally when most down-stream circuits have 0.4s anyway on their OCPD.

In the TT case you have two other factors:

• A fault will likely raise the whole earth system to close to live voltage (as Ze/Ra is much higher than the supply R1) - so you really don't want that to persist too long
• Due to this high Zs, you have no real options other than RCD to meet disconnection on an earth fault

Once you see that RCD are pretty much your only choice in the TT case then the times make more sense:

• 0.2s is enough for instant/personal protection RCD
• 1s allows for a delay RCD to have selectivity with downstream RCD/RCBO

 

[Flanders]

It's also different for TT systems, just to complicate matters.

For a TN system it's 0.4 seconds for final circuits up to 32A (without socket outlets) or up to 63A where there are socket outlets (this includes the industrial sockets mentioned by Flanders that can be rated at 63A).

In most domestic situations you won't generally see a socket circuit above 32A as you say.

On a TN system, for any other distribution circuit or circuit not covered by the first rules 5 seconds is permitted.
Just to add if the TT system has all the bonding in place and protected by an over current device then you use the disconnection times for a TN system
For a TT system, the times are 0.2 seconds and 1 second respectively.

This is all assuming 230V AC - they are different again for higher voltages.

The most common case (TN) for the 5s disconnection is on a sub-main, there are two reasons for having a longer ADS time:

• Potentially better selectivity with downstream OCPD (generally with a fused sub-main feeding a sub-board with MCB) where you would design for as high a fuse rating as safe (ADS time & adiabatic limits)
• On high current circuits the supply Ze might not allow a faster time in any practical choice of cable, etc.

Generally when most down-stream circuits have 0.4s anyway on their OCPD.

In the TT case you have two other factors:

• A fault will likely raise the whole earth system to close to live voltage (as Ze/Ra is much higher than the supply R1) - so you really don't want that to persist too long
• Due to this high Zs, you have no real options other than RCD to meet disconnection on an earth fault

Once you see that RCD are pretty much your only choice in the TT case then the times make more sense:

• 0.2s is enough for instant/personal protection RCD
• 1s allows for a delay RCD to have selectivity with downstream RCD/RCBO

Just to add for TT system if all the bonding is in place and there is over current protection for the circuit (so most installations) then the TN disconnection values are used.

 

[Mark Wright]

I asked a similar question recently and think it was Ian1981 who answered but there is another question I wanted to ask.

If you look at Table 41.1

and Cmin - UK voltage can go up by 10% so 240v and 250v TN systems have a disconnection time of 0.2 seconds ? is that correct ?

0.07s TT? not a figure I've ever heard.

 

[pc1966]

If you look at Table 41.1

and Cmin - UK voltage can go up by 10% so 240v and 250v TN systems have a disconnection time of 0.2 seconds ? is that correct ?

That table is for the nominal voltages to Earth. So the typical UK domestic and most industrial LV supply is defined as 230V Uo (but +10% -6% range) hence it falls in to the 2nd column for Uo <= 230V

The 0.07 TT time would be for something unusual like a 400V phase-to-neutral / 690V phase-phase supply for big industrial stuff.

 

[Mark Wright]

That table is for the nominal voltages to Earth. So the typical UK domestic and most industrial LV supply is defined as 230V Uo (but +10% -6% range) hence it falls in to the 2nd column for Uo <= 230V

The 0.07 TT time would be for something unusual like a 400V phase-to-neutral / 690V phase-phase supply for big industrial stuff.

makes sense, seems so obvious now?‍, Thanks

 

[Dartlec]

Just to add for TT system if all the bonding is in place and there is over current protection for the circuit (so most installations) then the TN disconnection values are used.

I've always read that as where disconnection is achieved by an overcurrent device rather than by an RCD, meaning that it only applies where the Zs is low enough to blow the fuse or trip the breaker, which is very rare in TT installs

Have I been reading that wrong? In most TT installs the RCD will trip first for a L-E fault and in that case disconnection is not being achieved by an overcurrent protective device.

 

[pc1966]

I've always read that as where disconnection is achieved by an overcurrent device rather than by an RCD, meaning that it only applies where the Zs is low enough to blow the fuse or trip the breaker, which is very rare in TT installs

That is the note on Table 41.1 where it states if it disconnect on OCPD and has all the supplementary bonding then the TN times apply.

If it walks like TN and quacks like TN...

That would be pretty rare! Though I guess it might apply to a low current circuit with a fuse or MCB in the 1A or so range and above-average earth rod conductivity, or maybe a steel frame building that has an exceptionally low Ra in the 1 ohm or less region depending on MCB choices, etc.

 

[Mark Wright]

never even saw that, so what's it saying? TT where ADS for earth fault can be archive by MCB then you can use TN times if you have Main Bonding?

but for short circuit it's stays the same, normal TT readings 0.2s

that might be totally wrong..........

 

[pc1966]

I think the point is if the TT situation is basically like TN, then the longer times ADS times (0.4s / 5s) are OK as you would not be at much risk of a high fault voltage being touchable.

I guess disconnecting on OCPD implies it pulls the live down a bit, and the supplementary bonding means you would have to be outside and in unfortunate contact to true Earth and a bonded item to experience the fault at all.

 

[Flanders]

I've always read that as where disconnection is achieved by an overcurrent device rather than by an RCD, meaning that it only applies where the Zs is low enough to blow the fuse or trip the breaker, which is very rare in TT installs

Have I been reading that wrong? In most TT installs the RCD will trip first for a L-E fault and in that case disconnection is not being achieved by an overcurrent protective device.

No I think I've been reading wrong. I was not thinking about fault current. as I read the over current part and did not make the connection. So it is saying if the over current device will disconnect connect the supply if there is a fault current then we can use the 0.4s value if not which as said is not going to happen in most cases .

 

[Dartlec]

No I think I've been reading wrong. I was not thinking about fault current. as I read the over current part and did not make the connection. So it is saying if the over current device will disconnect connect the supply if there is a fault current then we can use the 0.4s value if not which as said is not going to happen in most cases .

Given how rarely RCDs are not needed in a TT installation, at least on domestic, then it is probably moot anyway. Though if the correct time is put into the certificate it at least shows anyone looking at it that you know what you are doing - though that probably only happens at assessment time or if something goes wrong.

 

[davesparks]

And also why would you have socket outlets on a 63A breaker when it should be on a 32A ring or 20A radial. .

Nothing says sockets 'should' be on any particular size breaker, they can be put on whatever size breaker is required.
There are standard circuits for 13A sockets which use 20A or 32A breakers.
But that doesn't mean you can only ever install sockets on those size breakers.

A 63A or 125A socket wouldn't do very well on a 32A MCB!
[automerge]1598123724[/automerge]

Some of is slightly moot other than correctly filling in certification, since RCD protection is now almost across the board in new installs so disconnection times are easily met.

No it isn't, RCD protection is not 'almost across the board' in new installs.
RCD protection is only required for the situations specified in BS7671.

It is only in domestic installations that RCD protection for every circuit has become the norm.