# 17th regs

Discuss 17th regs in the Industrial Electrician Talk area at ElectriciansForums.net

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#### daisy

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hi
can anyone help with equation in new regs page 59. reg 415.2.2
does R < 50 V/ Ia mean volts divided by amps. eg

for a 30 mA rcd. 50 divided 30 = 1.6ohms
or
for a 10 amp type b mcb. 50 divided 50 = 1.0 ohm

Daisy

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B

#### Bane

Help daisy out with this one!!!

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#### Shakey

hi
can anyone help with equation in new regs page 59. reg 415.2.2
does R < 50 V/ Ia mean volts divided by amps. eg

for a 30 mA rcd. 50 divided 30 = 1.6ohms
or
for a 10 amp type b mcb. 50 divided 50 = 1.0 ohm

Daisy
hi Daisy

the Ia means current causing operation of the protective device, so for a 30mA RCD, its IdeltaN which is 30mA

so divide 50 by 0.03 and you get 1667 ohms

now this figure is relevent trhoughout the regs

and there is a table of these figures in Part 4 (afew pages past the Max ZS tables)

So for example, a TT system protected by a 30mA RCD wold have a maximum rod impedence of 16667ohms

in a bathroom, if everything is on a 30mA rcd, and meets the disconnection times, you can dispense with supplementary bonding provided that any extraneous conductive parts odf have a resisrance of 1667 ohms or less to the main bonding

etc, etc

and what it all means is:

with a resistance of 1667 ohms, when a fault current of 30mA flowing (which is when the RCD trips) the fault voltage will be at 50V which is the maximum 'safe' level

job done

D

#### daisy

Hi - thanks for your replies, I'm having trouble sending a reply. The site is telling me I'm not logged in when I am, then I lose the message. I will try again later.
thanks Daisy.

Hi. Thanks again for your replies. Looking at shakey's answer to the equation confirms my doubts about this regulation. This reg (page 59) reg 415.2.2 is for checking the effectiveness of supplimentary equipotential bonding between accessable exposed conductive parts and extraneous conductive parts and the equation given is as above. This calculates, as Shakey has shown to 1667 ohms. (well, don't know about you but if I have cross bonded two accessable parts I would hope the resistance would be lower than that). In the old 16th edit the value recommended was 0.05 ohms or less. This regulation is also used in the locations containing a bath or showerpage 166 reg 701.415.2.See the note at the bottom of this reg.

Last edited by a moderator:
S

#### Shakey

Hi - thanks for your replies, I'm having trouble sending a reply. The site is telling me I'm not logged in when I am, then I lose the message. I will try again later.
thanks Daisy.

Hi. Thanks again for your replies. Looking at shakey's answer to the equation confirms my doubts about this regulation. This reg (page 59) reg 415.2.2 is for checking the effectiveness of supplimentary equipotential bonding between accessable exposed conductive parts and extraneous conductive parts and the equation given is as above. This calculates, as Shakey has shown to 1667 ohms. (well, don't know about you but if I have cross bonded two accessable parts I would hope the resistance would be lower than that). In the old 16th edit the value recommended was 0.05 ohms or less. This regulation is also used in the locations containing a bath or showerpage 166 reg 701.415.2.See the note at the bottom of this reg.

it is NOT for checking the effectiveness of the supplementary bonding!

what it means is, for a bathroom, if extraneous conductive parts have a resistance of 1667 ohms or less to the main bonding AND all circuits on a 30mA RCD AND they all meet their disconnection times then supplemtary bonding is not required.

exposed conductive parts are already connected through the CPC

so if there is an earth fault on a bathroom circuit, the extraneous conductive parts can not reach a fault voltage higher than 50V before the RCD trips

and yes, it would normally be a lot lower!

and remember, if the resistance is a LOT higher (i.e. open circuit) then the chances are there is plastic pipework and it isnt an extraneous conductive part anyway!!! (and should NOT be supplementary bonded)

D

#### daisy

Hi
Thanks Shakey.
I see what you mean by this not checking the supplementary bonding in this case.
Right then try this!
In that note (page166) which we have already mentioned we are asked to check between the extraneous parts and the main earthing terminal.As you have said this could be 1667 ohms what happens if it is a T.T system with an earth electrode resistance, including earthing conductor of say 100 ohms measured from the main earthig terminal. this would now take us above the max value of in this case a 30 mA RCD.
Thanks Daisy

S

#### Shakey

Hi
Thanks Shakey.
I see what you mean by this not checking the supplementary bonding in this case.
Right then try this!
In that note (page166) which we have already mentioned we are asked to check between the extraneous parts and the main earthing terminal.As you have said this could be 1667 ohms what happens if it is a T.T system with an earth electrode resistance, including earthing conductor of say 100 ohms measured from the main earthig terminal. this would now take us above the max value of in this case a 30 mA RCD.
Thanks Daisy
ok Daisy, i can see your chain of thought, but you may becoming...erm..'confused' between earthing and bonding, as so many people do!!

so for a TT, protected by a 30mA, max Zs of 1667 ohms means it will disconect within the 50V

BUT of course the Zs is Ze (which we know is external) + (R1 + R2)

and of course R2 is the cpc resistance for your circuit from its extremity to the MET

and we know this is earthing, and is the fault path the earth fault current will take, and will take into account the exposed conductive parts

however, as you also no doubt know, the extraneous conductive parts are all about bonding, not earthing, and of course the bonding, (main or supplementary) are not considered to part of the earth fault current path

so consider an earth fault on a bathroom circuit (even on a TT system)

so a fault current is flowing through the exposed conductive part, through the CPC and back to the MET (and when it reaches a max of 30mA the RCD pops)

so lets say you are touching the extraneous conductive part (say a radiator) and the now live exposed condcutive part at the same time

if they were cross bonded they would be at the same potential, and you would be safe.

and if the resistance of the extraneous to the MET was say 1000ohms, then the fault current would have to decide to go the 'easy way' back through the low resistance R2 to the MET or the 'hard way' through YOU AND the 1000ohms extraneous path.

And of course, the exposed conductive part cant get above 50V fault voltage anyway, so you now have a max of 50V AND a 'high' resistance path between you and an electric shock. And of course this only until the RCD trips!

so your 100 ohms TT analogy would ONLY come into play IF there was an earth fault AND the CPC went open circuit

and of course, the definition of fault protection includes the words "under SINGLE fault conditions"

so yes, theoretically, IF the CPC failed, AND your extraneous had a resistance to the MET of 1667ohms AND the Ze through the earth electrode was 100 ohms AND (finally!) there was an earth fault then the fault voltage could rise marginally above 50V before the RCD tripped

BUT

to 'feel' that 50V you would have to be touching the extraneous AND exposed conductive parts at the same time

AND we would now be outside the realms of fault protection (the reason the RCD was put there in the first place) because you would be under multiple fault conditions

hope this has helped

oh, and as with most of my posts

i just made all that up.......

D

#### daisy

Thanks Shakey.
Just what i thought ???
I like your point on the " single fault condition ".This is a term i have skipped over not paying any attention to.
I am working my way through the new regs so i am sure i will have more problems.
Thanks Daisy.

S

#### Shakey

Thanks Shakey.
Just what i thought ???
I like your point on the " single fault condition ".This is a term i have skipped over not paying any attention to.
I am working my way through the new regs so i am sure i will have more problems.
Thanks Daisy.
you get any more problems Daisy, fire away