UK RCD Tripping on Zs Test?

[CalB4676]

Hey,

Apricate any thoughts on this issue,

Installed Power Out To External Out Building, New SWA 3 Core 4mm, Short Run. FuseBox 4-Way RCD MS Consumer Supplying 1 Light Circuit, 2 Double Socket On A Radial In Small Shed. Supplied From Main DB (Older Crabtree Starbreaker Consumer).

New Installation Tested Out Perfectly, R1+R2 On All Circuits At Shed DB and SWA Supply. Insulation Resistance Also Clear On All. Issue I'm Running Into Is When Attempting To Do A Zs Test On Anything On The Load Side Of The New Sub Main RCD. It Trips The RCD.

My First Thought Was Earth Leakage Problem, Insulation Resistance On Everything New Is Clear. Tried Disconnecting All Neutrals At Main DB and Neutral On RCD Load Side At Sub Main. So Only Testing From Supply Through DB 1 Out To Said Troublesome RCD At Sub Main. But Still Trips When Carrying Out Zs On RCD Load Side.

Ze At Main DB 0.11 Ohms (0.05ohms L-N), PFC 2.18Ka (4.20Ka L-N)

Zdb At Sub Main 0.17ohms / 1.38Ka

RCD At Sub Main Trip Times 117ms x1, 21.4ms x5
RCD Ramp Test Shows 23mA Trip Current

I'm Using A Megger MFT 1720 Using The 3 lead-low (No-Trip) Setting. I Know Testers Can Introduce Earth Leakage However Never Had This Issue Of My Tester Tripping An RCD Every.

With Knowing What the Zdb and R1+R2 Of The Circuits I Can Calculate Zs To Ensure Values Are Acceptable. RCD Trip Times Both x1 And x5 Are Within Expected Values. So On Paper Everything Is Correct However Something Not Sitting Right With Me With That RCD And Not Being Able To Measure A Zs.

Thinking It Could Be A Issue With The RCD. Have Two Fusebox RCBOs On A Different Job Showing Massively Different Trip Times To The Rest Also, So Make Me Wonder How Good They Are.

 

[westward10]

Yes calculate and note on the Certificate they were calculated.

 

[newfutile]

Nothing wrong with calculating ,i recently had trouble with my kewtech tripping eaton rcbo, the manufacturer told me how to turn off the pre test function which fixed the problem.
The tester works by putting current down the cpc ,never tested a fusebox make db with main (RCD) Residual Current Device to know if those times are typical , i thought it was only hagar that are that slow.

 

[westward10]

It is how the 239 is taught now the only live Zs is done at a socket outlet anything that requires the leads apart from Ze is calculated.

 

[EricMark]

Out of interest, what was the background leakage? Remember, the insulation tester uses DC, and with AC there is always some capacitive and inductive linking, so you need to measure the leakage with 50 Hz AC.

My own house is just within the 9 mA limit for a single 30 mA RCD, but would not dream of doing that, it has 14 RCBO's.

The ramp test on my RCD tester seems to use stages, and most RCD's seem to show 27 mA but never worked out what the stages are, so could well be 23 - 27 mA so the 23 mA could be 19 - 23 mA as said never worked it out.

I think there should be a space to show background leakage, so we can see if getting worse. Old house had two old Wylex fuse boxes fed from two 30 mA RCDs rather old, fitted second hand in around 1992, and one would always trip with the loop impedance test, at that time my clamp meter only had 10 mA increments so don't know what the background leakage was.

When I got the new clamp on, it took some hunting to find one with 1 mA increments and DC, the AC is great, but the DC needs to be zeroed, and the slightest movement can affect the reading, so measuring 6 mA DC is really a bit hit-and-miss. Higher currents, no problem. I look at all the clamp-on meters with 10 mA increments and wonder who buys them? When I got my first one in Hong Kong when working there, we were not using RCDs like we do today, it still works, but note not the same reading as new one.
May be it was time to upgrade, got a feeling the jaws were not closing fully on the yellow one, but we calibrate the loop impedance tester, but when do we do our multi-meters?

 

[Risteard]

Hey,

Apricate any thoughts on this issue,

Installed Power Out To External Out Building, New SWA 3 Core 4mm, Short Run. FuseBox 4-Way RCD MS Consumer Supplying 1 Light Circuit, 2 Double Socket On A Radial In Small Shed. Supplied From Main DB (Older Crabtree Starbreaker Consumer).

New Installation Tested Out Perfectly, R1+R2 On All Circuits At Shed DB and SWA Supply. Insulation Resistance Also Clear On All. Issue I'm Running Into Is When Attempting To Do A Zs Test On Anything On The Load Side Of The New Sub Main RCD. It Trips The RCD.

My First Thought Was Earth Leakage Problem, Insulation Resistance On Everything New Is Clear. Tried Disconnecting All Neutrals At Main DB and Neutral On RCD Load Side At Sub Main. So Only Testing From Supply Through DB 1 Out To Said Troublesome RCD At Sub Main. But Still Trips When Carrying Out Zs On RCD Load Side.

Ze At Main DB 0.11 Ohms (0.05ohms L-N), PFC 2.18Ka (4.20Ka L-N)

Zdb At Sub Main 0.17ohms / 1.38Ka

RCD At Sub Main Trip Times 117ms x1, 21.4ms x5
RCD Ramp Test Shows 23mA Trip Current

I'm Using A Megger MFT 1720 Using The 3 lead-low (No-Trip) Setting. I Know Testers Can Introduce Earth Leakage However Never Had This Issue Of My Tester Tripping An RCD Every.

With Knowing What the Zdb and R1+R2 Of The Circuits I Can Calculate Zs To Ensure Values Are Acceptable. RCD Trip Times Both x1 And x5 Are Within Expected Values. So On Paper Everything Is Correct However Something Not Sitting Right With Me With That RCD And Not Being Able To Measure A Zs.

Thinking It Could Be A Issue With The RCD. Have Two Fusebox RCBOs On A Different Job Showing Massively Different Trip Times To The Rest Also, So Make Me Wonder How Good They Are.

If you try changing the loop test to DC mode then this can prevent tripping of some RCDs.