Voltage Drop

[NoSparks]

We are all familiar with the requirements for measuring Zs and verifying that it is within the limits for the particular type of protective device. I use a single guide of 1.2 ohms as the maximum though there are of course many variations.

What we (I) have not been noting is (R1+ Rn) and the implicite voltage drop associated with it.

A quick calculation shows that if Zs for a ring circuit were 1.2, and we assume a generous 0.20 for Ze,
Then R1+R2 = 1.0, and assuming the circuit wired with 2.5/1.5 then R1 = 0.4 so (R1+Rn) = 0.8.

Now 0.8 ohms at 32A gives a voltage drop of 25.6 volts. MORE THAN DOUBLE that allowed for power circuits.

So should we be looking for a maximum Zs of 0.48 ohms to ensure the voltage drop requirement is satisfied?

 

[Des 56]

If we use a standard ring circuit,we are constrained by the length we are permitted
I believe the standard ring circuit limits the length, to ensure that voltage drop is within limits without needing calculation to prove it

Table 7.1 of the on site guide, gives max lengths where voltage drop is the limiting factor

 

[NoSparks]

Its a good point but I don't believe the area limit will keep the Zs down. I have just counted the number of rings I have tested and found that ~ 60% have Zs > 0.5 ohms.

 

[JUD]

We are all familiar with the requirements for measuring Zs and verifying that it is within the limits for the particular type of protective device. I use a single guide of 1.2 ohms as the maximum though there are of course many variations.

What we (I) have not been noting is (R1+ Rn) and the implicite voltage drop associated with it.

A quick calculation shows that if Zs for a ring circuit were 1.2, and we assume a generous 0.20 for Ze,
Then R1+R2 = 1.0, and assuming the circuit wired with 2.5/1.5 then R1 = 0.4 so (R1+Rn) = 0.8.

Now 0.8 ohms at 32A gives a voltage drop of 25.6 volts. MORE THAN DOUBLE that allowed for power circuits.

So should we be looking for a maximum Zs of 0.48 ohms to ensure the voltage drop requirement is satisfied?

The maximum length of a ring wired in 2.5/1.5mm² (limited by voltage drop) is 106m from table 7.1 OSG

4*(11.5 * 1000) / 26 / 18 / 0.923 = 106m (Type B BS 60898 MCB)

This gives you a max Zs of 1.42Ω at 70°C or 1.13Ω when tested at 20°C (worst case TN-S) to ensure you are within voltage drop limits. Obviously, the lower your Ze is, the lower the max Zs will become with regards to voltage drop.

106 * ((19.51 / 1000) * 1.2) / 4 + 0.8 = 1.42Ω (70°C)

1.42 * 0.8(rule of thumb) = 1.13Ω (20°C)

This satisfies the max Zs value of 1.44Ω (70°C) / 1.15Ω (20°C) from table 41.3 of BRB

 

[JUD]

Also just worked out that an R1+R2 of 1.0Ω on a ring circuit gives you a circuit length of 215m....

(1 x 4) ÷ (19.51 ÷ 1000) = 215m

...which is more than double what the maximum length from Table 7.1 OSG is for a ring circuit, and as such the voltage drop is more than double.