Discuss Back to basics (confused myself) in the Electrician Courses : Electrical Quals area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

gazdkw82

-
Arms
Reaction score
735
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?
 
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.
 
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.
 
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?
 
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?
 
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).
 

Reply to Back to basics (confused myself) in the Electrician Courses : Electrical Quals area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock