Continuity of CPC - Maximum reading
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sparkytim
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Hi Damien.
No not exactly, the value you obtain from continuity testing is either R2 or
R1 +R2.
If you add the R1 + R2 to the Ze Reading then you have Zs
Tables in the Regs Give MAXIMUM Zs Values for different types of Protective Devices that enable them to Automatically Disconnect the Supply (ADS) within the TIME required for that circuit.
For example You have a Radial Circuit to a Cooker wired in T&E and run in Mini Trunking.
This does NOT require RCD protection as it has No 13A socket just a Cooker switch, and the wiring is NOT embedded in the wall.
NO RCD is requiered so ADS can be provided by a 32A Type B MCB to BS 60898
You have obtained an R1 +R2 Reading of 1.00 ohms for this circuit (Its a long one) wired in 6mm T&E Cable.
Will it comply? Well if the Ze value for this installation is 0.35 ohms see if you can find out.
BRB Pg 49 Table 41.3 gives You the Maximum Zs value, however just to complicate matters we have to Multiply this by 0.8, to see if our TEST value is ok.
Let us know what you have worked out ( NO HELP PLEASE)
No not exactly, the value you obtain from continuity testing is either R2 or
R1 +R2.
If you add the R1 + R2 to the Ze Reading then you have Zs
Tables in the Regs Give MAXIMUM Zs Values for different types of Protective Devices that enable them to Automatically Disconnect the Supply (ADS) within the TIME required for that circuit.
For example You have a Radial Circuit to a Cooker wired in T&E and run in Mini Trunking.
This does NOT require RCD protection as it has No 13A socket just a Cooker switch, and the wiring is NOT embedded in the wall.
NO RCD is requiered so ADS can be provided by a 32A Type B MCB to BS 60898
You have obtained an R1 +R2 Reading of 1.00 ohms for this circuit (Its a long one) wired in 6mm T&E Cable.
Will it comply? Well if the Ze value for this installation is 0.35 ohms see if you can find out.
BRB Pg 49 Table 41.3 gives You the Maximum Zs value, however just to complicate matters we have to Multiply this by 0.8, to see if our TEST value is ok.
Let us know what you have worked out ( NO HELP PLEASE)
Theorysparky
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Damien
what are your readings for your R1,,Rn ?
and i assume you are on a 32amp mcb type b ?
what are your readings for your R1,,Rn ?
and i assume you are on a 32amp mcb type b ?
Theorysparky
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Doh
sorry mate i should read the posts properly !!!!! :coat
JUD
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The best way to do it is determine what the maximum length of the cable is for your maximum Zs AND Voltage Drop.
You can do this using these equations:
Zs
Z41 - Ze / (R"1 + R"2) * Cr
where
Z41 = Tabulated max Zs values from BS 7671
Ze = Measured external earth loop impedance
R"1 + R"2 = Resistance of your line and cpc conductors in mohms/m
Cr = correction factor for conductor operating temp.
So for your pretend cooker circuit the calculation would be:
1.44 - 0.35 / (10.49/1000) * 1.2 = 124m
Voltage Drop
(Max VD * 1000) / Ib / mV/A/m
where
Max VD = Maximum Voltage Drop
Ib = Design Current
mV/A/m = Voltage Drop in millivolts per Amp per meter
So the calculation would be:
11.5 * 1000 / 32 / 7.3 = 49m
So the maximum length for a 32A cooker circuit on 6mm
You can do this using these equations:
Zs
Z41 - Ze / (R"1 + R"2) * Cr
where
Z41 = Tabulated max Zs values from BS 7671
Ze = Measured external earth loop impedance
R"1 + R"2 = Resistance of your line and cpc conductors in mohms/m
Cr = correction factor for conductor operating temp.
So for your pretend cooker circuit the calculation would be:
1.44 - 0.35 / (10.49/1000) * 1.2 = 124m
Voltage Drop
(Max VD * 1000) / Ib / mV/A/m
where
Max VD = Maximum Voltage Drop
Ib = Design Current
mV/A/m = Voltage Drop in millivolts per Amp per meter
So the calculation would be:
11.5 * 1000 / 32 / 7.3 = 49m
So the maximum length for a 32A cooker circuit on 6mm
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In addition to Sparkytims info..Hi guys,I've just been looking into what the maximum allowed value is when testing for continuity of CPC but I can't seem to find one - Is there one?
The other consideration you could make if you have just done a wander lead method to get an R2 value on its own...
Is a quick calc using the values from Table 9A pg 166 OSG to see if the Calculated cable length is approx to your calculated length...
e.g.
Say you have run a 2.5mm radial for an immersion heater..
by looking at the physical installation you know the circuit is approx 20m - 25m fusebox to Immersion heater cupboard.
2.5mm T&E has 1.5mm CPC
1.5mm conductor is 12.10mohm per meter
so your MAX R2 value should be approx 0.24ohms to 0.30ohms.
If you were getting readings significantly higher then this it would imply something wrong..
either with you reading method... or bad joint... or damaged cable..
Where are you getting the figures in red from?In addition to Sparkytims info..The other consideration you could make if you have just done a wander lead method to get an R2 value on its own...
Is a quick calc using the values from Table 9A pg 166 OSG to see if the Calculated cable length is approx to your calculated length...
e.g.
Say you have run a 2.5mm radial for an immersion heater..
by looking at the physical installation you know the circuit is approx 20m - 25m fusebox to Immersion heater cupboard.
2.5mm T&E has 1.5mm CPC
1.5mm conductor is 12.10mohm per meter
so your MAX R2 value should be approx 0.24ohms to 0.30ohms.
If you were getting readings significantly higher then this it would imply something wrong..
either with you reading method... or bad joint... or damaged cable..
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Page 166 table 9A On Site Guide...
third line down....
a 1.5mm conductor on its own .. 12.10 milliohms per metre.
NOT a line & CPC value
e.g.
you can see on the 6th line down a 2.5mm conductor is 7.41 milliohms
and therefore a 2.5/1.5 T&E is 12.10+7.41= 19.51
(which is as written in the 8th line down!)
the 20 & 25 are just my assumed example cable lengths...
:coffee
If you test continuity of CPC by testing R1 + R2 would you use the value in the 8th line down (19.51)?Page 166 table 9A On Site Guide...third line down....
a 1.5mm conductor on its own .. 12.10 milliohms per metre.
NOT a line & CPC value
e.g.
you can see on the 6th line down a 2.5mm conductor is 7.41 milliohms
and therefore a 2.5/1.5 T&E is 12.10+7.41= 19.51
(which is as written in the 8th line down!)
the 20 & 25 are just my assumed example cable lengths...
:coffee
sparkytim
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Well thats moved on a bit since lunch time.
So hope thats all helped Damien, coming back to your original question Continuity test results can be used to check various things and give some helpfull information to assist in determining complience with the Regs.
So hope thats all helped Damien, coming back to your original question Continuity test results can be used to check various things and give some helpfull information to assist in determining complience with the Regs.
