Back to basics (confused myself)

[gazdkw82]

Decided to have a browse through onsite guide and something struck me with disconnection times. Maybe I'm missing something obvious but onsite guide and bs7671 states that a disc time of not more than 5 secs is permitted for:

Final circuits exceeding 32A.

So does that mean a cooker/shower/motor etc basically any fixed equipment over 32 A is permitted to anything upto 5 seconds?

Am I missing something obvious?

 

[The Ghost]

No that is the case. It is usually for high resistive loads such as cookers and showers, not so sure about the motors though that's in its own class of protection.

 

[happysteve]

Yep, that's right.

5 seconds (on a TN system) for:
- distribution circuits of any current
- socket circuits greater than 63A
- final circuits (not incorporating a socket) over 32A

There's some interesting reading in the "Commentary on the IET Wiring Regs" book about the disconnection times in table 41.1, and how they came about. Nothing about the 5 second limit though. I suspect it involved either a beer mat, or a smoke packet.

 

[Baddegg]

Yep, that's right.

5 seconds (on a TN system) for:
- distribution circuits of any current
- socket circuits greater than 63A
- final circuits (not incorporating a socket) over 32A

There's some interesting reading in the "Commentary on the IET Wiring Regs" book about the disconnection times in table 41.1, and how they came about. Nothing about the 5 second limit though. I suspect it involved either a beer mat, or a smoke packet.

Are you saying @telectrix wrote the regs?

 

[gazdkw82]

It's just a little shocked (no pun intended) that something like a shower can have a 5 sec disc time.

RCD would come into its own in that scenario however.

 

[Strima]

It's just a little shocked (no pun intended) that something like a shower can have a 5 sec disc time.

RCD would come into its own in that scenario however.

What's the likelihood of a person causing a dead short between line and neutral whilst the circuit is in normal use?

 

[gazdkw82]

What's the likelihood of a person causing a dead short between line and neutral whilst the circuit is in normal use?

I'm not sure exactly and I'm not sure where you are going with the question?

 

[Bobster]

I'm not sure exactly and I'm not sure where you are going with the question?

The safety of the regulations have to take some practicality into the equation.

The protection afforded to meet the 5s rule is not to protect a person. Only the circuit. usually due to the sizing of cables etc, they can often stand the fault current for up to 5s safely.

There's also discrimination etc.. to take into account.

Start looking at HV networks, 5 seconds is normally the minimum disconnection time. (After being in a substation when a 2MVA 33/11K V transformer shorted it's secondary winding's, 5 seconds is a hell of a long time to hear it roar and the foundations shake before the protection took over).

 

[gazdkw82]

Thanks Rob.

That's really made it clear

 

[Bobster]

Thanks Rob.

That's really made it clear

It's still early and I haven't had my coffee yet!

Does it make sense to you?

 

[gazdkw82]

It's still early and I haven't had my coffee yet!

Does it make sense to you?

Yes absolutely. I think I was confusing a short fault with earth faults and applying that to the disc times. So in my head the disc time were insinuating an electric shock upto 5 secs is compliant