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

 

[IQ Electrical]

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

Yes but they would likely be on a 'A1 Ring Final Circuit with 32A protective device so would have a TN system disconnection time of 0.4 seconds.

 

[snowplough]

Hi IQ,

Am in right in thinking then ,that for a L-E fault in the above scenario with the lawn mower for instance , the 0.4 s disc time is there to prevent elec shock and not cables being damaged causing fire etc. If this is the case then why in the bs7671 on page 46 ,note 1 at bottom of page says ,disconnection is not req for protection against elec shock but may be required
for other reasons such as thermal effects prevention?.

This is whats confusing me i think

kind Regards

LOCKET

 

[IQ Electrical]

Basic protection is the usual form of 'basic' shock protection ie. insulation of live parts etc.

Protection under fault conditions ensures that accessible conductive parts ie. metallic enclosures, pipes etc. must not be hazardous under single fault conditions.


ADS is used to limit the magnitude and duration of fault voltage between exposed, conductive and extraneous-conductive parts.


Where shock risk is increased (special locations etc.) then additional protection is required ie. RCD protection or supplementary bonding.
This also guards against failure of any of the basic/fault protection measures.

 

[Chr!s]

The 0.4 sec is part of ADS and is based on Biegelmeier and Lee's testing of human thresholds at 230 v.

Not sure about the 5 sec, but the bigger the circuit the larger the fault current required to disconnect, on large installations it will be difficult to generate enough current to disconnect within 0.4, so allowing upto 5 secs give some leeway.

 

[Chr!s]

Hi IQ,

Am in right in thinking then ,that for a L-E fault in the above scenario with the lawn mower for instance , the 0.4 s disc time is there to prevent elec shock and not cables being damaged causing fire etc. If this is the case then why in the bs7671 on page 46 ,note 1 at bottom of page says ,disconnection is not req for protection against elec shock but may be required
for other reasons such as thermal effects prevention?.

This is whats confusing me i think

kind Regards

LOCKET

Cables need to be protected against thermal effects using the adiabatic, now for short circuit time needs to be limited to ensure limiting temp of cable is not exceeded.

Now for a earth fault current there are two requirements, first we have ADS so disconnection times need to be met, secondly we again need to ensure that the adiabatic equation is met. Alternatively you may use table 54.7(but note second paragraph 543.1.1)