100mA Type S RCD as Main Switch on TT Zs of 188 Ohms wouldn't trip.

[greenspark]

I was testing the above RCD yesterday on my Megger MFT1553 using the Autotest function set to the correct trip range of 100mA but I couldn't get a response from the RCD even at x1. At this point, the meter would simply flash <50V. I then retried the test using the manual functions but still got the same result. I know that the Earth Fault Path was terrible but I tested a 30mA RCD on the same installation and got it to operate successfully through all trip currents.
What I am concerned with is whether:

1.
It was the meter or the person operating it causing this problem,
I.E: I held down the blue 'degree' button to try to change the RCD Type to S (Time Delay) but it didn't seem to give me a clear S option.

OR,

2.
A 100mA Type S RCD actually needs a better earth fault path to operate within time than a 30mA RCD or possibly just in this case due to this particular 100mA RCD. I suppose to find out the latter I would need to try replacing the RCD for like with another.

Interested to know your thoughts on this, I know I've gone to town on this message but knowledge is power. An apt description for our trade I reckon.

 

[trebor]

A 100mA RCD need an Ra of 167 or below to operate in the given time thats why the instrument indicates above 50 V

Ra = 50/Idn

 

[Deleted member 26818]

You won't be able to conduct the auto tests on this type of RCD.
For one the RCD is time delayed, so will not trip in the given time, and secondly, the 5X will not test, as the current required is 500mA.

 

[greenspark]

Thanks spinlondon.
Sometimes the answer is staring you in the face and it still gets missed! So is there any way of testing a time delay RCD that you know of?

 

[Deleted member 26818]

I'm not that conversant with the Megger, I use a Metrel MI3000.
However from what I recall, with the Megger in manual and the correct mA selected, connect the leads or plug adapter and with a voltage reading showing in the display, you then hold in the blue button until a warning triangle comes up above the voltage reading. Then test away.

 

[greenspark]

Yep, thats what I I tried to no avail, I'll see what literature I can find for my meter, I'm sure there's a demo dvd kicking about that came with it. Thanks again though

 

[SPARTYKUS]

Should've slummed it and got a 1552 like me, works fine for me

 

[tony mc]

As spin says but make sure your not on DC range.
​

Time Delayed (Selective) RCDs​

The selective RCD test is enabled by a long press on the blue degree
button. As selective RCD test mode is enabled the MFT will emit a
long beep and the display will show the exclamation mark (!)
symbol. Once selected perform the RCD tests in the same way as
normal RCDs (see RCD testing above).
To disable Selective RCD mode, press the degree button for two
seconds.
​

0º or 180º

On either the​

​

​

​

I, 5I or DC test, to switch between 0º and 180º​

press the BLUE 0°/180° button.

 

[tony mc]

A 100mA RCD need an Ra of 167 or below to operate in the given time thats why the instrument indicates above 50 V

Ra = 50/Idn

167 Ohms is that not for a 300 mA RCD

50/0.100 = 500

 

[Guest123]

A 100mA RCD need an Ra of 167 or below to operate in the given time thats why the instrument indicates above 50 V

Ra = 50/Idn

Not quite, 50V/0.10mA = 500ohms.

 

[SPARTYKUS]

Not quite, 50V/0.10mA = 500ohms.

I'm pushing my luck here with Lenny being a moderator but

it should be 50V/0.10A = 500 ohms (not 0.10mA thats very very small)

 

[Guest123]

Whats a small m between friends.

 

[SPARTYKUS]

Love it.

 

[greenspark]

Haha, yeah, testing on my 1553 is like trying to drive the deathstar!

 

[telectrix]

Whats a small m between friends.

as long as the m don't stand for morrisons, lenny LOL.