No Trip Zs at light switch

[Hefferl]

I use a Megger 1553. Can someone explain how I can test the Zs at a light switch where no neutral is present and the circuit is rcd protected.

I cant use the 3 lead No Trip method because I cannot connect to a neutral.

Am I forced to measure R1 + R2 and then calculate Zs (ie Ze + R1 + R2)

Thanks.

 

[Ash]

Why are you doing it at the switch?

 

[S60TEM]

try connecting the black neutral lead to the earth lead. If that still doesn't work, do the R1+R2 then calculate.

 

[Hefferl]

Why are you doing it at the switch?

It could be the electrically furthest point of the circuit and thus the highest Zs

---------- Post Auto-Merged at 13:07 ---------- Previous post was made at 13:05 ----------

try connecting the black neutral lead to the earth lead. If that still doesn't work, do the R1+R2 then calculate.

This will still cause an imbalance between the current flowing in the line and neutral and will trip RCD

 

[rugbymacc]

move mcb over to non rcd side, for the test if possible

 

[Hefferl]

move mcb over to non rcd side, for the test if possible

Thanks but it's a dual RCD CU with no non-RCD ways available.

 

[Doc Hudson]

It could be the electrically furthest point of the circuit and thus the highest Zs

As the live still has to make its way to the light fitting after the switch, it is general the fitting that is further along the circuit than the switch. As such I understand the general place for taking your R1+R2 or Zs would be the light fitting. Additionally as you have pointed out you cannot take a reading requiring a neutral when there is no neutral to connect on to.

Doc H.

 

[Theorysparky]

Hefferl

First question is why are you doing a zs reading ?????

not a trick question is it after a cu upgrade ?? an alteration to the lighting cct ?? PIR ???

Doc is correct in stating it should be at the ceiling rose at the end of line if you can find it,this way you can check polarity of the cct

switch live and neutral with lamp removed

does your meggar have a trip lock ???? if so your reading may be higher but well within limits

why not do a Ze+(R1+R2)= Zs calculation ????

 

[Hefferl]

HefferlFirst question is why are you doing a zs reading ?????

not a trick question is it after a cu upgrade ?? an alteration to the lighting cct ?? PIR ???

Doc is correct in stating it should be at the ceiling rose at the end of line if you can find it,this way you can check polarity of the cct

switch live and neutral with lamp removed

does your meggar have a trip lock ???? if so your reading may be higher but well within limits

why not do a Ze+(R1+R2)= Zs calculation ????

Its after replacing CU - on reflection I agree with you and Doc - rose will always be further. My thinking was if I measured Zs at every point I could establish highest Zs for circuit whilst also confirming presence of CPC at each light and switch. Thanks for everyone's input.

 

[Theorysparky]

This is why i do all the testing before i change the cu so no need to zs the lights afterwards

if the r1+r2 are ok then and the ze is good no need for zs

by doing r1+r2 on the lights before the cu change you find the highest reading anyway

dont think the furthest point is the highest reading

tip...

zs your sockets before you change cu you get a lower reading......pick one socket and zs it after the cu change the difference between the readings will be your triplock loss

 

[ianmacd]

In fact, H&S wise, R1+R2 + Zdb would be safer than doing live testing with potentially exposed live parts up a ladder/steps/etc...

 

[a1]

If you go by H&S, it will be a lot safer if you don't do any electrical work at all. :_|

:coat

 

[ianmacd]

:yellow card

 

[Evans Electric]

If you go by H&S, it will be a lot safer if you don't do any electrical work at all. :_| :coat

Ahhh!! Admin seems to be reading my thoughts , that answer is definitely in Deke style.

 

[philbas]

If you go by H&S, it will be a lot safer if you don't do any work at all. :_| :coat

Sorted for you

 

[Andy™]

try connecting the black neutral lead to the earth lead.

that wont work. the neutral is used to take most of the current so it doesnt trip the RCD. stick neutral lead onto earth and you may aswell do a 2 lead test. its just going to trip

only options is to move to non-RCD (even bypass RCD - just dont forget to un-bypass it), or calculate

 

[mapleford]

The ceiling rose should always the furthest point but if you really want to test at the switch try turning the switch off, red on permanent live , green on cpc and black on switched live.

 

[springcrocus]

I still think the easiest way to test Zs on any circuit that might give you the RCD blues is to do a 2-lead test at the board. This method bypasses the RCD so you don't need to worry about tripping it. You don't need a neutral, either.

For those who are not sure how to do this, I have outlined the procedure below.

1. RCD's in the OFF position

2. All MCB's in the OFF position

3. Place a shorting link from line to earth at the point you want to test.

4. Turn the MCB for that circuit to the ON position.

5. Green test lead on the outgoing line terminal of the relevant RCD

6. Red test lead on the incoming line terminal of the same RCD

7. There's your reading.

8. Turn OFF the MCB and remove the shorting link

Hope this helps someone. ; \

 

[ADS]

You beat me too it, Springcrocus, so I'll second it.

 

[PC Electrics]

I use a Megger 1553. Can someone explain how I can test the Zs at a light switch where no neutral is present and the circuit is rcd protected. You can'tI cant use the 3 lead No Trip method because I cannot connect to a neutral. Correct

Am I forced to measure R1 + R2 and then calculate Zs (ie Ze + R1 + R2) Yes

Thanks.

I use a 1552.

If you just want to check there is earth continuity at switches, connect a wander to the MET and probe to the screw heads.

Max R1+R2 (or max Zs if you prefer) will always be at the light, not the switch.