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.

 

[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!
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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.

 

[Dartlec]

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.

Yep, should have clarified I was talking about domestic as I did in my later post.

 

[ForkEnigma]

You can hav

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 have 40A electrack feeding sockets