Please Explain Dissconection times

[acestu]

Hi could somebody please explain dissconection times in simple terms coz i am thick, ie what circuits are .4 and which are 5 secons and why...

thanks so much
Acestu

 

[Mr Mark Sparks]

short answer

tt installations .2

tns tncs .4 for everything upto 32a except submains

5secs for submains and (i think ) circuits over 32a

 

[acestu]

Thanks for the reply mark, could you tell me why the times are so....

thanks
acestu

 

[Mr Mark Sparks]

i dont know really who says what times for what things
or why there isnt just one time for all
especially as max eli readings are listed for .4 and 5 secs as the same
sorry cant help more

 

[Des 56]

Its to do with the potential of the situation and what harm could come to a person or animal

Dis connection times are provided so that the supply to a faulty circuit can be dis connected before harm comes to that person
Medically there is a time period when the heart gives up the ghost,the giving up bit depends on the way the fault is distributed through the body

If someone was barefoot in a bathroom touching a live pipe with their hands, and their foot was touching a radiator that was conducting the current to earth,then the person would be a bit on the dead side if the current was not cut off in a particular time

If that same person did the same in a bedroom .it would be unilikely to be in barefoot on a wet floor or touching a pipe and the path would not be so conductive,so less likely to give up the ghost

The dis connection times reflect these reasons
To make a overcurrent device operate in these times there are maximum impedance values that need to be met
The reason for this is that a small current can flow for an eternity before it operates,but a large current can make it operate very quickly
So its better to have a mammoth flow of current for a short period than a smaller current for a longer period
In order for that massive flow of current the circuit needs an easy fault path with very low resistance in order for that to be possible

 

[Mr Mark Sparks]

hi des
i know what you mean
i just wonder why "they" decided to change most of the old 5sec stuff ie lights and fixed appliances down to .4 with the 17th
the risk hasnt changed
only thinking out loud really

 

[adam.h]

hi des
i know what you mean
i just wonder why "they" decided to change most of the old 5sec stuff ie lights and fixed appliances down to .4 with the 17th
the risk hasnt changed
only thinking out loud really

health and safety

 

[Deleted member 9648]

"They" decide to change things for no apparent reason on a regular basis because there are a vast number of "They"s and they have to do something all day to justify their existence.

 

[acestu]

Thank you for your comments everybody, i,m just a student trying to learn here, and your help is well appreciated..

So i take it that that there is a table in the new 17th regs somewhere that lists dissconnection times for certain circuits using certain dissconection devices.

thanks again
Acestu