pre CU change testing

[lloyd1980]

Hi all - I posted this in the Cu changes and you thread but have had no replys, wondering if someone might pick it up in here and give me an answer.
its in relation to performing an IR test on an exsisting CU to show any potential problems with an RCD / RCBO setting when first energised.
This is before performing a CU change
Thanks in advance

A great thread BTW

I have glanced at this as about to price for a CU change
Just a quick one, domestic 30ma RCBO . Will trip when there is a 30ma imbalance between the line and neutral conductors

If I was to do an IR test at 500V dc between neutral and earth prior to install;

0.00 would indicate a short and would then indicate definate fault once installed

my question is this, if your Ir readings are low but above 1M ohm ( min ac ceptable reading )

at what point would the RBCO start to register a fault???? eg if the IR reading are low 2.6Mohm or in region of this fig, would an acceptable IR reading still trip the RCBO?

obviously its not a short but its showing signs of deterioration - at what point would the RCBO trip, when there is a short only????
I have known RCBO's to trip without there being a short, just preasure faults where conductors are kinked as been stuffed behind accesories.

just wondering does anyone have a definative answer to this
Thanks

( this is asuming all IR tests are coming back low )
​

 

[Smugley]

How about a supposition to start off..

Voltage / trip current = 230V / 0.03A = 7666 Ohms. Or 0.0076 MOhms.

The limit of 1 MOhm might produce an earth leakage of 230/1M = 0.23 mA

 

[Sintra]

Also one to remember is that a 30mA RCD can trip at anywere between 16mA and 30mA.

 

[IQ Electrical]

Hi all - I posted this in the Cu changes and you thread but have had no replys, wondering if someone might pick it up in here and give me an answer.
its in relation to performing an IR test on an exsisting CU to show any potential problems with an RCD / RCBO setting when first energised.
This is before performing a CU change
Thanks in advance

A great thread BTW

I have glanced at this as about to price for a CU change
Just a quick one, domestic 30ma RCBO . Will trip when there is a 30ma imbalance between the line and neutral conductors

If I was to do an IR test at 500V dc between neutral and earth prior to install;

0.00 would indicate a short and would then indicate definate fault once installed

my question is this, if your Ir readings are low but above 1M ohm ( min ac ceptable reading )

at what point would the RBCO start to register a fault???? eg if the IR reading are low 2.6Mohm or in region of this fig, would an acceptable IR reading still trip the RCBO?

obviously its not a short but its showing signs of deterioration - at what point would the RCBO trip, when there is a short only????
I have known RCBO's to trip without there being a short, just preasure faults where conductors are kinked as been stuffed behind accesories.

just wondering does anyone have a definative answer to this
Thanks

( this is asuming all IR tests are coming back low )
​

I think we tend to forget that 1 MOhm is 1 million Ohms!

In my experience, most RCD tripping faults after board changes come from appliances with earth leakage that was there previously but remained undetected pre RCD.

Try a plug-in RCD unit on older/suspect appliances, it's surprising how many suffer from some leakage and also, your client can see that you're not hiding a fault on your own work!