Why is R1 not r1....?

[swaRRR]

When we measure R1 + R2, we're essentially saying R1 is resistance of the line, and R2 is resistance of the CPC, and we add them together.

But then when we measure them individually we call them r1 and r2. If we're taking the two measurements and adding them together and calling them R1 R2 why aren't they referred to as their individual measurements, r1 and r2?

The R1 refers to the resistance of the line in the equation (R1 + R2) but the r1 refers to the resistance of the line too. So why do we name the same thing two different ways? Couldn't we technically just call it r1 + r2? Same thing, no?

Hope this makes sense because i've confused myself. Which isn't hard.

 

[timhoward]

Your first sentence is correct.
Small r1, rn and r2 normally notate the end-to-end continuity measurements of ring final circuits.

 

[Julie.]

When we measure R1 + R2, we're essentially saying R1 is resistance of the line, and R2 is resistance of the CPC, and we add them together.

But then when we measure them individually we call them r1 and r2. If we're taking the two measurements and adding them together and calling them R1 R2 why aren't they referred to as their individual measurements, r1 and r2?

...

Hope this makes sense because i've confused myself. Which isn't hard.

No we don't, they are completely different things.

R1, Rn, and R2 are all measurements of the line, neutral, and earth/cpc resistances as they present to a fault.

So if you have a cable from the cu to a single point (lamp, outlet etc) if you measure the resistance this would be R1, Rn, R2.

However, with a ring circuit if you measure the end to end resistance (from the cu round the ring, back to the cu) this is NOT the same as the resistance a fault will see.

A fault on the furthest socket it's R1 would be formed by that part of the ring down one side, in parallel with the other part of the ring.

So if you measured r1 as 4 ohms (end to end), if the furthest socket is only 1/2 way around the ring, then it would be only 1/2 of 4 ohms (2 ohms) to the socket - BUT since each side would be 2 ohms back to the cu, you have two 2 ohms in parallel. So 1 ohm.

In this case r1 = 4 ohm, but R1 = 1 ohm.

 

[pc1966]

At first I found the R1/r1 difference confusing as many folks don't see any difference as in many cases for typing it is too much bother to caps-shift!

 

[pc1966]

Also as @Julie. has said the equivalent R1+R2 for a RFC is (r1+r2)/4 in the worst-case at half way around the ring.

It is one of the reasons the RFC can cover such an area with moderate cable use, subject of course to total load bing below 32A and the loads being reasonably distributed around the ring.

 

[swaRRR]

No we don't, they are completely different things.

R1, Rn, and R2 are all measurements of the line, neutral, and earth/cpc resistances as they present to a fault.

So if you have a cable from the cu to a single point (lamp, outlet etc) if you measure the resistance this would be R1, Rn, R2.

However, with a ring circuit if you measure the end to end resistance (from the cu round the ring, back to the cu) this is NOT the same as the resistance a fault will see.

A fault on the furthest socket it's R1 would be formed by that part of the ring down one side, in parallel with the other part of the ring.

So if you measured r1 as 4 ohms (end to end), if the furthest socket is only 1/2 way around the ring, then it would be only 1/2 of 4 ohms (2 ohms) to the socket - BUT since each side would be 2 ohms back to the cu, you have two 2 ohms in parallel. So 1 ohm.

In this case r1 = 4 ohm, but R1 = 1 ohm.

If i measure a piece of wire, with no CU involved though, I don't get how measuring R1 + R2 (line and earth added together), can mean the R1 is denoted differently to if we just measure the r1 alone.

I guess my question is, i take a piece of cable say just on my kitchen worktop.

I attach line and cpc together at one end and measure the two other ends. This is R1 (line resistance) and R2 (cpc resistance) added together, correct?

So if i then measure just line end to end, why can this measurement not be the same as the R1 from the other test? Why is it denoted differently? That's what i don't get. I understand there are other factors at play when we do it on a system but from a pure mathematical symbol perspective why are the R1 and r1 different? Isn't the 'other part of the ring' technically just the other end of the wire once we remove them from the CU?

 

[pc1966]

If i measure a piece of wire, with no CU involved though, I don't get how measuring R1 + R2 (line and earth added together), can mean the R1 is denoted differently to if we just measure the r1 alone.

R1 and/or R2, R1+R2 is the resistance to the potential fault location. Worst case for a radial is the end of the wire.

r1 and/or r2, r1+r2 are also "end of wire" for the RFC but they are looped to the start, so the worst-case fault for which R1+R2 is used is then half way along the wire.

 

[swaRRR]

R1 and/or R2, R1+R2 is the resistance to the potential fault location. Worst case for a radial is the end of the wire.

r1 and/or r2, r1+r2 are also "end of wire" for the RFC but they are looped to the start, so the worst-case fault for which R1+R2 is used is then half way along the wire.

RIGHT that makes sense thanks pal.

 

[Julie.]

If i measure a piece of wire, with no CU involved though, I don't get how measuring R1 + R2 (line and earth added together), can mean the R1 is denoted differently to if we just measure the r1 alone.

I guess my question is, i take a piece of cable say just on my kitchen worktop.

I attach line and cpc together at one end and measure the two other ends. This is R1 (line resistance) and R2 (cpc resistance) added together, correct?

So if i then measure just line end to end, why can this measurement not be the same as the R1 from the other test? Why is it denoted differently? That's what i don't get. I understand there are other factors at play when we do it on a system but from a pure mathematical symbol perspective why are the R1 and r1 different? Isn't the 'other part of the ring' technically just the other end of the wire once we remove them from the CU?

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 .

 

[Intoelectrics]

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.

 

[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.