Why ? 120-130v old tester

[Dustydazzler]

So, I was messing around yesterday with some old testers I have laying around to see if they still work

I found my dads old Robin MFT and noticed something I have never noticed before

When you test of the EFLI setting put live and neutral probes on a switch or socket but don't connect the earth probe you get 120-130 ish volts.
Soon as you clamp the earth lead on it jumps back to 230-240V

Why is this ?

 

[newfutile]

my kewtech does the same thing i treat it as an error message , i expect there is a potential divider in the meter

 

[Dustydazzler]

I assumed some sort of fail safe. Its a 3 lead tester and if you don't connect the earth lead to either the CPC or onto the N you get the red warning light which I knew happened but never ever noticed a voltage reading of around 120-130v.

 

[Mikegh]

I tried an old Robin 3-wire fault loop tester

Leaving off the earth shows 0 on the LCD display

P-N neon indicator is fully lit

N-P neon reverse indicator is partially lit

 

[Dustydazzler]

I tried an old Robin 3-wire fault loop tester

Leaving off the earth shows 0 on the LCD display

P-N neon indicator is fully lit

N-P neon reverse indicator is partially lit

The old Robin MFT I have if you put the red probe on live and black probe on teh neutral you get 120-130V showing on the display but the RED warning light is on and if you press the test button nothing happens. When you put the green lead and clamp it on the CPC it jumps to 240v and the green light come on and you can procede with test

 

[timhoward]

The dear old Robin and Kyoritsu testers (and early Kewtech) do the high current loop by comparing "loaded" mains with unloaded mains.
A 10 ohm load is added between Line and Earth for a couple of +ve cycles. This charges up a certain capacitor.
Neutral to Earth (i.e. unloaded mains) then charges up a different capacitor for a couple of +ve cycles.
(There's also a wave rectifier making sure the charging happens very briefly and only during a couple of positive cycles to avoid overheating the relays or the load resistors)
Then the two capacitor voltages are compared and the loop impendence is estimated using the calibration constants.

They therefore need a 3 wire supply, and that's why you see the 120v if one of the returns isn't there.
This method was still used as late as the Kewtech KT63, the earth and neutral leads are externally joined to do a 2 wire loop test.

(Megger instead use a relay to switch in and out the test load and can use the same 'return' for both measurements, so their 2 wire hi-current doesn't need a Neutral)

 

[Mikegh]

The dear old Robin and Kyoritsu testers (and early Kewtech) do the high current loop by comparing "loaded" mains with unloaded mains.
A 10 ohm load is added between Line and Earth for a couple of +ve cycles. This charges up a certain capacitor.
Neutral to Earth (i.e. unloaded mains) then charges up a different capacitor for a couple of +ve cycles.
(There's also a wave rectifier making sure the charging happens very briefly and only during a couple of positive cycles to avoid overheating the relays or the load resistors)
Then the two capacitor voltages are compared and the loop impendence is estimated using the calibration constants.

They therefore need a 3 wire supply, and that's why you see the 120v if one of the returns isn't there.
This method was still used as late as the Kewtech KT63, the earth and neutral leads are externally joined to do a 2 wire loop test.

(Megger instead use a relay to switch in and out the test load and can use the same 'return' for both measurements, so their 2 wire hi-current doesn't need a Neutral)

How do you know all this ?

 

[westward10]

 

[timhoward]

How do you know all this ?

I have a dangerous fascination for wanting to know how things work. In this case I got hold of some service manuals which for this age of gear are generally well written documents.
One of the reasons I still use a 1553 tester is that I have the 15 series service manual. I've been trying to get hold of a 17 series service manual for years now!