Couple of questions, r1r2, Zs and polarity

[mak]

On a ring main we test r1r2 at every outlet since there is no end of the circuit and every test should return same-ish figure unless there is a loose connection or socket is on a spur, right?
What about the the radial circuit? Other than to check polarity is there a point to test anywhere other than at the switch i.e. end of the line? Unless the reading at the end of the line is higher than expected?

Zs which is earth fault loop (R1+R2+Ze), we test the impedance to make sure in case of a fault on a circuit the protective device will react before the wire will melt into soup?
I remember there was a formula that explained this, something like design current ≥ conductor current ≥ fault current.

An appliance with a double pole switch fed from an outlet with crossed line and neutral will not get damaged unless it is polarity sensitive, if there is one?

Thanks.

 

[pc1966]

On a ring main we test r1r2 at every outlet since there is no end of the circuit and every test should return same-ish figure unless there is a loose connection or socket is on a spur, right?

Exactly, not only are you verifying polarity you also find any dodgy sockets in the process. Typically you will see very small differences in R1+R2, of the order of 0.05 ohm or less, but anything significant should be checked. If it is a spur adding a little no problem, but if not it could be a bad connection or bad socket.

Remember at 13A a resistance of 0.1 ohms will be dissipating 13*13*0.1 = 16.9W which in a localised area is going to burn something our quickly!

What about the the radial circuit? Other than to check polarity is there a point to test anywhere other than at the switch i.e. end of the line? Unless the reading at the end of the line is higher than expected?

If it is a radial of sockets then you ought to check each socket as well, it is just you will see R1+R2 increase as you move away from the CU towards the end of the line.

Zs which is earth fault loop (R1+R2+Ze), we test the impedance to make sure in case of a fault on a circuit the protective device will react before the wire will melt into soup?
I remember there was a formula that explained this, something like design current ≥ conductor current ≥ fault current.

You might want to check that order of comparison!

You have several issues:

• Cable and OCPD ratings have to be above the normal operating current
• Cable size may need to be increased due to thermal effect: if insulated, if in a hot zone, if bundled with other heat-generating wires
• Cable size may also need to be increased if voltage drop is too large
• The Zs value has to be low enough so the OCPD will disconnect sufficiently fast for both shock safety, and to keep the fault I2t low enough to avoid cable damage/fire risk.
• If the load is not fixed (e.g. a circuit with sockets on it so you have no control over what is used later) the OCPD must also protect the cable against overload so the "operating current" for cable calculations becomes the OCPD rating, not the planned load.

An appliance with a double pole switch fed from an outlet with crossed line and neutral will not get damaged unless it is polarity sensitive, if there is one?

NO NO NO! Reversed polarity is very dangerous because you have the fuse in a 13A plug only in the line conductor. If L&N are swapped at the socket then any N-E fault in the appliance will have to take out the 32A MCB or similar and it is unlikely the flex can carry such current without a major fire risk.

That is why reversed polarity is a C1 fault on inspection, up there with exposed live conductors, as needing immediate correction.

 

[mak]

Cable and OCPD ratings have to be above the normal operating current

• Cable size may need to be increased due to thermal effect: if insulated, if in a hot zone, if bundled with other heat-generating wires
• Cable size may also need to be increased if voltage drop is too large
• The Zs value has to be low enough so the OCPD will disconnect sufficiently fast for both shock safety, and to keep the fault I2t low enough to avoid cable damage/fire risk.
• If the load is not fixed (e.g. a circuit with sockets on it so you have no control over what is used later) the OCPD must also protect the cable against overload so the "operating current" for cable calculations becomes the OCPD rating, not the planned load.

Cable rated higher than design current makes sense, in fact I was told on occasions by sparks(allegedly) that the bigger the better...

Yes there is more to selecting the size of a cable than a design current, but they way I understand it boils down to the economy, you want to "go less not more" while not compromising on safety.

If L&N are swapped at the socket then any N-E fault in the appliance will have to take out the 32A MCB or similar and it is unlikely the flex can carry such current without a major fire risk.

Whoa forgot about the fuse. That makes a lot of sense! There was a rare case on site once, painters were mixing their paint and used 110v extension lead from a transformer, the lead got burned all the way along its length, it could be that the mixer plug had reversed polarity and had a short or a voltage spike and the lead couldn't take it in time for the breaker on the transformer to react.

 

[Julie.]

AS PC ^^

However - what are you testing for?

Is it upon installation, in which case 100% testing is required at all outlets and points, not just on RFC outlets

If it's an EICR, then you can do less than 100% at whatever reasonable % agreed with the client

 

[Lucien Nunes]

There was a rare case on site once, painters were mixing their paint and used 110v extension lead from a transformer, the lead got burned all the way along its length, it could be that the mixer plug had reversed polarity and had a short or a voltage spike and the lead couldn't take it in time for the breaker on the transformer to react.

Polarity is probably not a factor here. 110V site supplies are centre-tapped to earth, i.e. there are two wires each 55V from earth, making 110V in total. Since neither is earthed, they are both lines, there is no neutral, and therefore no polarity. Double-pole switches and breakers are required (although small portable transformers are often protected by a single-pole breaker on the 230V input side)

A more likely cause of the burnout was that the cable was not rated for the OCPD that was feeding it (e.g. 16A extension fed from 32A outlet via unfused splitter) and/or the cable was too long and the loop impedance too high to clear the fault promptly.

 

[Julie.]

Polarity is probably not a factor here. 110V site supplies are centre-tapped to earth, i.e. there are two wires each 55V from earth, making 110V in total. Since neither is earthed, they are both lines, there is no neutral, and therefore no polarity. Double-pole switches and breakers are required (although small portable transformers are often protected by a single-pole breaker on the 230V input side)

A more likely cause of the burnout was that the cable was not rated for the OCPD that was feeding it (e.g. 16A extension fed from 32A outlet via unfused splitter) and/or the cable was too long and the loop impedance too high to clear the fault promptly.

I assume the issue of polarity here is the fuse upstream of the transformer, as often there is actually no protection on the 110v side although I appreciate the downstream device was mentioned.

More likely to be overload though granted.

 

[Lucien Nunes]

Good point, I should have said more explicity: Polarity of the 110V wiring...

 

[radiohead]

On a ring main we test r1r2 at every outlet since there is no end of the circuit and every test should return same-ish figure unless there is a loose connection or socket is on a spur, right?
What about the the radial circuit? Other than to check polarity is there a point to test anywhere other than at the switch i.e. end of the line? Unless the reading at the end of the line is higher than expected?

Zs which is earth fault loop (R1+R2+Ze), we test the impedance to make sure in case of a fault on a circuit the protective device will react before the wire will melt into soup?
I remember there was a formula that explained this, something like design current ≥ conductor current ≥ fault current.

An appliance with a double pole switch fed from an outlet with crossed line and neutral will not get damaged unless it is polarity sensitive, if there is one?

Thanks.

It is important to get your terminology correct to avoid confusion. r1, r2 (and rn) are ring circuit end to end readings and not the same as R1+R2.

 

[mak]

AS PC ^^

However - what are you testing for?

Is it upon installation, in which case 100% testing is required at all outlets and points, not just on RFC outlets

If it's an EICR, then you can do less than 100% at whatever reasonable % agreed with the client

Yes I meant upon installation, I know inspection reports are not as straightforward.

Polarity is probably not a factor here. 110V site supplies are centre-tapped to earth, i.e. there are two wires each 55V from earth, making 110V in total. Since neither is earthed, they are both lines, there is no neutral, and therefore no polarity. Double-pole switches and breakers are required (although small portable transformers are often protected by a single-pole breaker on the 230V input side)

Didn't know that.

It is important to get your terminology correct to avoid confusion. r1, r2 (and rn) are ring circuit end to end readings and not the same as R1+R2.

Yes I am aware of the terminology, thanks!

 

[radiohead]

On a ring main we test r1r2 at every outlet since there is no end of the circuit and every test should return same-ish figure unless there is a loose connection or socket is on a spur, right?

Yes I am aware of the terminology, thanks!

Best to use it then!

 

[mak]

Best to use it then!

Fair play sir

 

[Mark.fardon]

Remember at 13A a resistance of 0.1 ohms will be dissipating 13*13*0.1 = 16.9W which in a localised area is going to burn something our quickly!



Really ???
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Remember at 13A a resistance of 0.1 ohms will be dissipating 13*13*0.1 = 16.9W which in a localised area is going to burn something our quickly!

Really ???

 

[pc1966]

Really ???

This is an example of a 15W resistor without a heatsink:

Ohmite 10Ω Wire Wound Resistor 15W ±5% TUW15J10RE | RS

uk.rs-online.com

It is 5cm long and runs at 200C above ambient.

This is an example of a 15W resistor designed for use with a metal heatsink:

Arcol, 100Ω 15W Wire Wound Chassis Mount Resistor HS15 100R J ±5% | RS

uk.rs-online.com

It is 2cm long, rated at full power when on a 1mm aluminium sheet of 415cm2 (about 20cm x 20cm) at which point the temperature rise above ambient is 75C.

 

[Paul L]

On the question of terminology, and i promise i'm not being pedantic, it is probably a ring final circuit you are testing rather than a ring main. It always seems to get called a ring main and in fairness most people would know what you mean but as a trainee its best to use the correct terms if you can.

When i was a boy an electrician said to me quite gruffly ' A ring main is something that feeds a housing estate and a ring final is a socket circuit usually in your house' and it always stuck in my head what he said.

As i said, i'm not trying to be clever and i probably need to get out more but i think its best to use industry terminology if possible.