testing

[snowplough]

Hi Guys,

Im at collage at the moment and ive loads of questions to ask.

First one is regardind testing, when you do a continuity test on the cpc on a lighting circuit , and you loop the line and earth at the consumer unit to form a loop, and the ou go in and out of ever light switch and rose, i take it you are making sure that the cpc is continually connected right back to the cons unit if their is an earth fault at that location i think?.

Secondly i am told that when you are are supposed to the furthest point in the circuit to do an R1 nad R2 test, say the last ceiling rose , i take it this is because this should have the highest resistance .

Now what im confused about is what about the strappers say in the two way and intermidiate switch because these dont seem to get tested, what im thinking is if you just find a live point say at an intermidiate switch and the earth when you do your contin for the cpc test ou are not testing another strapper ,so if the one ou dont test is fault for some reason and has a very high resistance this would really be your true R1 and R2 reading if its the highest on the circuit.

This leaves me very confused and im sure there is an easy answer im not seeing.

Many thanks

Snowplough

 

[johnnyb]

The reason you go to the farthest fitting is to prove the cpc return path, knowing if you have it there its got to go back ok , and that mcb will trip according to regs.
Regarding not testing the 2way, who says this, of course you test it and in a domestic install its some times the farthest away due to the strappers, so you must test that mate.

 

[pushrod]

when R1&R2 testing two way /intermediate lighting make sure you operate the switches so that you end up testing all possible paths.

 

[snowplough]

Many thanks for the reply Pushrod,

So what i was thinking was if the earth lead actually terminated in the last ceiling rose then if you put your two testing probes to the earth and the line in the last ceiling rose on the circuit and flicked all the switches so that the current went through all the possible paths it can through all the strappers then surel that would be one possible way of getting our R1 reading and testing for continuity. Or the other way if the earth terminates in the last switch, if it two way or intermidiate then if like you say if you put one of the probes on earth in each switch and then you tested all the four pieces of strappers then you would also be checking continuity of cpc, and also testing the line continuity b checking the strappers as above and if this reading is higer then the ceiling rose reading then this would be your new R1 and TR2 reading if i am right.

Look forward to your reply

Many Thanks

Snow plough

 

[pushrod]

Many thanks for the reply Pushrod,

So what i was thinking was if the earth lead actually terminated in the last ceiling rose then if you put your two testing probes to the earth and the line in the last ceiling rose on the circuit and flicked all the switches so that the current went through all the possible paths it can through all the strappers then surel that would be one possible way of getting our R1 +R2 reading and testing for continuity. Or the other way if the earth terminates in the last switch, if it two way or intermidiate then if like you say if you put one of the probes on earth in each switch and then you tested all the four pieces of strappers then you would also be checking continuity of cpc, and also testing the line continuity b checking the strappers as above and if this reading is higer then the ceiling rose reading then this would be your new R1 and TR2 reading if i am right.

Look forward to your reply

Many Thanks

Snow plough

Having problems visualising things exactly (few glasses of wine here )Think you are all correct in what you are saying and yes your highest reading is your R1+R2 - think you should always find this at one of your light fittings.

 

[Sintra]

Your R1 + R2 has to be taken at last light on circuit between switched live and earth. This test has to be repeated with the swiches in all combinations. Testing at the switch doesn't cover the full length of the circuit.

 

[snowplough]

Many Thanks Push rod and Sintra,

Sintra, when you said R1 and R2 is taken at the the last light on the circuit ,i take it you meant in the actuall ceiling rose at the very end of the line wire where it goes into the bulb, and the earth. If this had two wa and intermidiate switches on it a well i take it you you mean't by still having your by testing at the last ceiling rose and operating all the switches in every combination ou would be testing all the strappers as aid before.

I take it though you still need to go in the switches to test the cpc for continuity if it has the last bit of earth coming from the last ceilng rose instead of terminating in the ceiling rose if you no what i mean.

Hope im making sense

Many thanks for your patience

Snow plough

 

[pushrod]

I take it though you still need to go in the switches to test the cpc for continuity

yes
readings should not be as big as at your last lamp though.

 

[Des 56]

You need to confirm the continuity of the cpc at all switches
If they were to be metalic then the cpc continuity is essential for that fault path
You need to confirm that continuity of the cpc all throughout the whole of the circuit
(this could be done with a trailing wire back to the earth terminal of the consumer unit)

By confirming the earth path using a link at the supply end and measuring between live and earth,the highest reading will also be the declared R1+R2 reading

 

[snowplough]

Cheers push rod,

When you said readings in switches should not be as high as the reading in the last rose , i am just wondering why this is?Is it something to do with the line running through the switches encounters more resistance in the switches ,or is it just the fact that the last rose is at the end of the circuit i.e longer the cable,the more the resistance. But then if the latter is the case then surely, the switch or switches that run off from the last ceiling rose could be as long or longer depending if for instance, the cpc runs from the ceiling rose and terminates at the lat switch/s if im making sense.

Im jut tring to get me head around the theor of it all,

Man Thanks

Snow plough

 

[pushrod]

Cheers push rod,

When you said readings in switches should not be as high as the reading in the last rose , i am just wondering why this is?Is it something to do with the line running through the switches encounters more resistance in the switches ,or is it just the fact that the last rose is at the end of the circuit i.e longer the cable,the more the resistance. But then if the latter is the case then surely, the switch or switches that run off from the last ceiling rose could be as long or longer depending if for instance, the cpc runs from the ceiling rose and terminates at the lat switch/s if im making sense.

bold is the correct answer - for the current to get to the lamp it must go through a path involving the switches. In the event of a fault to earth the current takes the shortest cpc route, which would probably not be via the switches.
R1+R2 has to be the biggest resistance reading so you know what the maximum resistance could be in the the event of a fault. You need this value if you were going to calculate Zs. If you haven't done this yet you will be soon.

edit : you had to test at the switches to establish cpc continuity, just in case there was a fault at a switch.

 

[snowplough]

Cheers Pushrod,

I understand that you have to have cont of your cpc at the switches in case of a fault so it gos to earth and trips the breaker as in the ceiling rose, i was just trying to work out if the last ceiling rose was the longest termination of line and cpc ,or would it be in the last switch/s feding that rose especially if as i said before the cpc wire terminates in the switch/s and not in the last rose where the line terminates?

many thanks

asnow plough

 

[pushrod]

Cheers Pushrod,

I understand that you have to have cont of your cpc at the switches in case of a fault so it gos to earth and trips the breaker as in the ceiling rose, i was just trying to work out if the last ceiling rose was the longest termination of line and cpc ,or would it be in the last switch/s feding that rose especially if as i said before the cpc wire terminates in the switch/s and not in the last rose where the line terminates?

many thanks

asnow plough

At the ceiling rose it will be the longest line but the cpc route will not necessarily be the same length - it will probs be shorter. It will still be the biggest R1+R2 combined reading, even if there could be a long cpc route to a switch. If you draw the diagrams out and actually follow the routes involved it might help if you can't get your head round it.

 

[snowplough]

Regards Pushrod,

I am looking in the on site guide and under the cont test on page 79 it actually shows a pictiure of the probe leads from the tester at the line and earth in the switch for the R1 and R2 reading and not in the ceiling rose so i am still wondering why they people sa the furthest point of a circuit is in the last ceiling rose if even in the on site guide it is taking the R1 and r2 reading from the light switch attached to the last rose. If i follow the leads like ou say on the diagram in the O.S.G there seems to be the line longer then the cpc if read taken at the rose, and the cpc is longer then the line if read is taken at the switch ,so to me the last switch/s on the last ceiling rose seem to be as far as each other if i am looking at the diagram correct.


Many thanks

Snow

 

[pushrod]

Hi Snow, just went to get my OSG to see what diagram you are looking at and discovered i have left it at college in the workshop - hope it not walked off . TBH it doesn't matter where in the circuit you get your highest reading as long as you record the highest as your R1+R2 . They would test at all points to check continuity of the cpc.