Continuity of CPC - Maximum reading

[Dambo]

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?

 

[sparkytim]

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)

 

[Dambo]

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)

Is the answer 1.44 x 0.8 = 1.152?

If so the Zs of 1.35 is less than the BRB figure of 1.44 but more than the 1.152 figure (80%) so does this make it unacceptable?

 

[Theorysparky]

Damien

what are your readings for your R1,,Rn ?

and i assume you are on a 32amp mcb type b ?

 

[Dambo]

Damienwhat are your readings for your R1,,Rn ?

and i assume you are on a 32amp mcb type b ?

1 Ohm, 32A MCB as suggested by sparkytim.

 

[sparkytim]

Is the answer 1.44 x 0.8 = 1.152?If so the Zs of 1.35 is less than the BRB figure of 1.44 but more than the 1.152 figure (80%) so does this make it unacceptable?

Unacceptable. Well done. So how could we make it acceptable?

 

[sparkytim]

1 Ohm, 32A MCB as suggested by sparkytim.

Hi Theroy

Just a hypothetical example given

Thanks

Tim

 

[Theorysparky]

Hi TheroyJust a hypothetical example given

Thanks

Tim

Doh

sorry mate i should read the posts properly !!!!! :coat

 

[ianmacd]

Dohsorry mate i should read the posts properly !!!!! :coat

I think we all suffer from that from time to time.

 

[Dambo]

Unacceptable. Well done. So how could we make it acceptable?

Use a bigger cable? Or find a shorter route from the MCB to the cooker switch?

 

[JUD]

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

 

[SPECIAL LOCATION]

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?

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..

 

[Dambo]

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..

Where are you getting the figures in red from?

 

[JUD]

Where are you getting the figures in red from?

(12.10/1000) * 20 = 0.24

(12.10/1000) * 25 = 0.30

 

[Dambo]

(12.10/1000) * 20 = 0.24(12.10/1000) * 25 = 0.30

The value should be 19.51, not 12.10 i.e. 2.5mm line conductor with a 1.5mm CPC

 

[JUD]

The value should be 19.51, not 12.10 i.e. 2.5mm line conductor with a 1.5mm CPC

This is the measurement for the cpc only

Table 9A OSG

1.5 = 12.10

Your original question was for Max. R2 value. 19.51 would be for R1 + R2

 

[SPECIAL LOCATION]

(12.10/1000) * 20 = 0.24(12.10/1000) * 25 = 0.30

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

 

[Dambo]

This is the measurement for the cpc onlyTable 9A OSG

1.5 = 12.10

Your original question was for Max. R2 value. 19.51 would be for R1 + R2

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)?

 

[JUD]

If you test continuity of CPC by testing R1 + R2 would you use the value in the 8th line down (19.51)?

Correct

As SL said:

R1 = Resistance of 2.5mm

 

[sparkytim]

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.