the magic 1667 ohms

Help Support Talk Electrician Forum:

stringy

Senior Member
Joined
Oct 19, 2008
Messages
148
Reaction score
0
ive read on a few posts today about the magic 1667 ohms as somebody put it lol

im still learning the trade and could somebody please explain to me what its all about?

thanks matt

 

steptoe

of course Im wrong, ask my wife™
Joined
Feb 13, 2008
Messages
24,337
Reaction score
117
Location
Gtr Manchester
its about a reference to a max Zs to allow a maximum 50v touch voltage during fault conditions if you have a 30mA RCD for protection.

its actually a bit of a red herring now that 17th is here cos you are not supposed to use this method to calculate your disconnection times anymore, and the values would be slightly different anyhow being based on 230v now.

its still a good rule of thumb to work to tho.

 

stringy

Senior Member
Joined
Oct 19, 2008
Messages
148
Reaction score
0
arr right cheers steptoe

im kinda with you, so its a maximum zs value on any circuit? and earting system?

still confused lol

 

steptoe

of course Im wrong, ask my wife™
Joined
Feb 13, 2008
Messages
24,337
Reaction score
117
Location
Gtr Manchester
only if protected by a 30mA RCD.

but basically yes, you can use this as a reference to any circuit, although as it would be a deviation from BS7671 you would have to note this on the cert and also find/show an alternative way of proving your disconnection times are within BS7671-2008 guidelines.

 

sparkyork

Electrician
Joined
Oct 17, 2008
Messages
145
Reaction score
0
under certain condition and under certain applications of the 17th edition. to be fair i wouldnt be able to sleep at night if i left a socket circuit with a Zs of 1660 ohms say. what happens if the rcd fails? spesh if its a protek!! lol

 

steptoe

of course Im wrong, ask my wife™
Joined
Feb 13, 2008
Messages
24,337
Reaction score
117
Location
Gtr Manchester
under certain condition and under certain applications of the 17th edition. to be fair i wouldnt be able to sleep at night if i left a socket circuit with a Zs of 1660 ohms say. what happens if the rcd fails? spesh if its a protek!! lol
would totally agree.

in all honesty stringy, unless you have a really good and total understanding of how to acheive your disconnection times AND stay legal when deviating from BS7671 then it is always advisable to stay within the regs and the quoted max Zs values.

 

stringy

Senior Member
Joined
Oct 19, 2008
Messages
148
Reaction score
0
thanks very much for the info guys, makes sense now

ive only been testing for a few months now but i have never got a zs reading that high.

thanks again

 

Nicky Tesla

Well-known member
Joined
Apr 30, 2008
Messages
1,638
Reaction score
0
I = V/R, R = V/R, 50v (max touch voltage)/0.03 (30mA) = 1666.6 ohms

the fact that we have moved from 240v to 230v does not make a difference here, cos max touch voltage is still 50v.

If there is a fault to earth on a piece of equipment, the voltage on that equipment will RISE from the original 0v upwards (this will take only fractions of a second). When this voltage has risen to 50v, if the resistance to earth on r2 is low enough, then at least 30mA will flow to earth and the RCD will trip in time. The voltage may rise still further than 50v but only due to the time it takes for the RCD to work.

 

Shakey

Member
Joined
Oct 17, 2008
Messages
56
Reaction score
0
might be worth you reading the 'max Zs' thread, this is being discussed in depth there:D

 

stringy

Senior Member
Joined
Oct 19, 2008
Messages
148
Reaction score
0
might be worth you reading the 'max Zs' thread, this is being discussed in depth there:D
ive been following it shakey, thinking to myself will i ever have that much knowledge haha :^O

 

GreekIslandLover

Senior Member
Joined
Oct 18, 2008
Messages
119
Reaction score
0
Pardon me for being wrong (I'm sure Shakey will hit me if I am!) but generally speaking, high Ze values are likely to happen mostly with TT systems. If you have TN or TNCS then you should (hopefully) have low Ze to start with, and therefore compliance with the maximum Zs of MCB's should be acheivable.

 

Shakey

Member
Joined
Oct 17, 2008
Messages
56
Reaction score
0
Pardon me for being wrong (I'm sure Shakey will hit me if I am!) but generally speaking, high Ze values are likely to happen mostly with TT systems. If you have TN or TNCS then you should (hopefully) have low Ze to start with, and therefore compliance with the maximum Zs of MCB's should be acheivable.
absolutely correct GIL

and i stressed in the 'max Zs' post that the 1667 ohms figure should not be used as a 'get out clause' for poor design or installation:)

 

Latest posts

Top