disconnection times

[snowplough]

Hi Guys,
Wonder if any of you could try and explain to me if the reason for disconnection times for fuses /proctective devices etc. As on one hand, i was told at college it is to protect the cable from catching fire etc, and then iget the impression the reason for having say 0.4 disconnection times limits for TN SYSTEMS etc , is to limit somebody getting elec shock wile holding someting faulty etc. But then when i look in bs7671 under disconnection times it says the disconnection times are not there for shock protection but to protect the cable from thermal effects.

Im confused,

Regards

LOCKET

 

[Sparky Ninja]

Overcurrent devices are there to protect the installation and property alone. It only takes 50mA to kill, that's what RCD's are for.

The 0.4 secs and 5 secs times are affected by the fault current levels, and it is these levels that have to be monitored for thermal withstand tolerance.

The times themselves are determined by weather the circuit is a distribution circuit, greater than 32A (In), or a combination of the two.

 

[snowplough]

Mny Thanks wilder,
Sorry for sounding stupid but are you saying that the 0.4 and 5 seconds is not to prevent a person from recieving electric shock but soley to prevent the cable from catching fire .

Regards

LOCKET

 

[Sparky Ninja]

Mny Thanks wilder,
Sorry for sounding stupid but are you saying that the 0.4 and 5 seconds is not to prevent a person from recieving electric shock but soley to prevent the cable from catching fire .

Regards

LOCKET

Well strictly speaking no, as they do contribute towards the method of protection 'ADS'. However the times given are not in relation to protecting us, they are in relation to protecting the installation.

 

[snowplough]

Many Thanks,

So why would upto 32a circuits have 0.4 s disconnection max ,and over 32a have upto 5s disconnection max , if its to do with upto 32a being for outdoor equipment etc then would that not make you think it would want to discoonect quick(ie 0.4 s)for a line to earth fault say metal work on a lawn mower say,to prevent electric shock.
Or is it nothing to do with the above and just to prevent the cables from setting on fire .

Regards

locket

 

[IQ Electrical]

Many Thanks,

So why would upto 32a circuits have 0.4 s disconnection max ,and over 32a have upto 5s disconnection max , if its to do with upto 32a being for outdoor equipment etc then would that not make you think it would want to discoonect quick(ie 0.4 s)for a line to earth fault say metal work on a lawn mower say,to prevent electric shock.
Or is it nothing to do with the above and just to prevent the cables from setting on fire .

Regards

locket

The general thought was that proximity would be less on circuits rated at >32A, these circuits would also be expected to be higher integrity supplies and also, I remember something regarding the difficulty in complying with earth loop impedance maximum values on say long distribution circuits etc.

I think it was a combination of the above!

 

[malcolmsanford]

The general thought was that proximity would be less on circuits rated at >32A, these circuits would also be expected to be higher integrity supplies and also, I remember something regarding the difficulty in complying with earth loop impedance maximum values on say long distribution circuits etc.

I think it was a combination of the above!

If I remember did the newest admendment to the BS 7671-2008 intend to include circuits exceeding 32amp be included into table 41.5, and is that now included.

 

[IQ Electrical]

If I remember did the newest admendment to the BS 7671-2008 intend to include circuits exceeding 32amp be included into table 41.5, and is that now included.

Do you mean the Corrigendum or the forthcoming Amendment 1?

If it's Table 41.1 then I haven't seen any changes to it?

 

[malcolmsanford]

To be honest I thought it was table 41.5, but it may have been 41.1 as they both deal with circuits not exceeding 32amps, it was just the 32 amp that stuck in my mind, but as it's been off line now for 5 months nearly I maybe wrong .................it would not be the first time I was wrong lol, and what is .4 between friends

 

[IQ Electrical]

To be honest I thought it was table 41.5, but it may have been 41.1 as they both deal with circuits not exceeding 32amps, it was just the 32 amp that stuck in my mind, but as it's been off line now for 5 months nearly I maybe wrong .................it would not be the first time I was wrong lol, and what is .4 between friends

There might have been something in the DPC but I didn't notice it in the pre-print version that I briefly scanned through the other week (I was more interested in Part 6 )

 

[malcolmsanford]

There might have been something in the DPC but I didn't notice it in the pre-print version that I briefly scanned through the other week (I was more interested in Part 6 )

Typical tester ...........only interested in themselves

 

[snowplough]

Hi Malcolm,

Im still confused,
What i am trying to get my head round is if the .4 disconnection time is to protect circuits overheating then why have it say for portable equipment say from socket outlets where you would think a quick disconnection in the event of a L-E fault say on the casing of an electric lawn mower would be paramount to prevent an electric shock or would 0.4 seconds still be long enough to fatal or cause serious injury?

Regards

Locket

 

[IQ Electrical]

Hi Malcolm,

Im still confused,
What i am trying to get my head round is if the .4 disconnection time is to protect circuits overheating then why have it say for portable equipment say from socket outlets where you would think a quick disconnection in the event of a L-E fault say on the casing of an electric lawn mower would be paramount to prevent an electric shock or would 0.4 seconds still be long enough to fatal or cause serious injury?

Regards

Locket

How many circuits normally feeding lawn mowers and portable equipment will be rated at 32A or greater?

 

[snowplough]

Hi IQ,
None?

 

[snowplough]

Sorry IQ i meant to say just the socket outlets in the home or if fed from a garage say