Why is R1 not r1....?

[swaRRR]

Whatever measurements you make become very dependent upon how they are used in a circuit.

If you measure the end to end resistance of a piece of 4mm^2 wire, it just it's resistance R, if it is used as a phase conductor it's R1, neutral Rn etc.

If this is used as part of a total radial circuit it could be R(cu to jointbox hidden in the wall) - forming part of the R1 of that circuit.

By convention it is standard to use uppercase and subscripts 1, n, and 2 for that part of the final circuit that represents the resistances a fault may see. Hence R1 etc.

You could measure R1 directly ( the most common) or calculate it from all the parts that make up that circuit eg.
R1 = R(cu to jointbox) + R(jointbox to isolator) + R(isolator to heater)

As explained above, r1 etc is usually reserved for ring circuits where the end to end resistance of the closed ring is not the same as the resistance from one point (cu) to the mid point .

Amazing explanation thanks.

 

[swaRRR]

Another way to think about it is, when you are measuring R1+R2 is you are trying to determine what the highest resistance path for current flow in the event of a fault is so to verify that the protection device is the correct rating and will always operate correctly within the maximum specified time. For a radial circuit this is simply the furthest point on the circuit (end of line). However for a ring circuit the furthest point is always going to be mid way. However since "mid way" can technically be any point on the ring then the measurement for R1+R2 at each point will (should) be the same resistance, though we tend to use the midway point in relation to the CPD as the furthest point. This is the way that we can verify a true ring circuit and also why its important to verify this since the circuit capability relies on multiple parallel paths totalling no greater than a maximum specified resistance (R1+R2) values as tabulated in the regs) . The r1 measurement only gives you the line end to end resistance of the least resistive path of that circuit, there could well be multiple cross connections or radials spurred off that could be missed during testing from this measurement alone.

In addition the purpose of maintaining a true ring circuit is important to avoid overloading of the circuit. If you imagine as e.g that in a standard 2.5 T&E FRC there is a break in the ring, potentially either of the legs could be subject to carrying the full circuit load demand which could be higher than the current carrying capacity of the cable, especially so if there are derating factors applied, yet the OCPD could remain energised. Thus exposing the cable to damage and worse failure/fire etc... In this instance finding and fixing the fault (most ideal) or derating the OCPD (assuming the circuit is safe to do so) is crucial.

Awesome cheers.

 

[timhoward]

Keep asking the questions. (And watch John Wards videos on testing, they are very clear!)

 

[swaRRR]

Keep asking the questions. (And watch John Wards videos on testing, they are very clear!)

Yeah i'm just getting into the testing side of things since it interests me more than installation.

Lots of questions, lots of learning to be done.

For any onlookers reading this, the 'Learn Electrics' channel on youtube is very good also.