TT / RCD testing

[Smart Sparks]

Hi there
Just a little advice required on RCD testing:
I did some Minor Works the other day and obtained thefollowing RCD test readings –
TT system
100amp 30mA RCD protecting the CCU.
Circuit tested was a 32a ring final circuit with 32aovercurrent protection MCB type b.
0 degress
X 0.5 = no trip
X 1 = 29.5ms
X 5 = 9ms

180 degrees
X 0.5 = no trip
X 1 = 18ms
X 5 = 16ms

I was expecting similar readings on the 180 degree test compared to the0 degrees test. Can someone shed any light on the difference between the 0& 180 degree test and whether these readings look normal?.

I assumed the difference is the stage of the waveform the test is takenat?.

Overall ELI at the socket under test was 29.5 ohms

Also I noticed the Main Earth to the installation is 10mm, although thecable is buried without mechanical protection?, having looked at table 54.1(p159) it would appear the cable should be 16mm. I assume even though the ELIis ok the cable should really be replaced?.

Your thoughts and advice would be appreciated

Regards
Smart Sparks

 

[plugsandsparks]

was anything plugged into the load when it was being tested ?
Did you take several readings ?
It wouldn't worry me as you take the highest reading anyway which is fine.

 

[spark 68]

As P&S ^^^ above,

This is perfectly normal and are typical readings, the way I understand it is that some RCD's are more sensitive on one half cycle than the other, maybe be some other posters can confirm this, or expand on the reasons why.

 

[IQ Electrical]

As P&S ^^^ above,

This is perfectly normal and are typical readings, the way I understand it is that some RCD's are more sensitive on one half cycle than the other, maybe be some other posters can confirm this, or expand on the reasons why.

That's it exactly

 

[SirKit Breaker]

As all of the above. Repeat the tests 10 times and you will get 10 different results. It complies with 415.1.1 Job done.

Cheers.........Howard

 

[telectrix]

main earthing conductor on a TTsystem is minimum 6mm if not mechanically protected. no need for 16mm

 

[Smart Sparks]

Thanks for the feedback.

It is possible something was plugged in, i assume that can have an effect then.

I feel better now i know the resuits sound ok. i am a little confused on the earth size as the table in the regs states 16mm in table 54.1. Just out of interest where does the 6mm come from?.

 

[IQ Electrical]

Thanks for the feedback.

It is possible something was plugged in, i assume that can have an effect then.

I feel better now i know the resuits sound ok. i am a little confused on the earth size as the table in the regs states 16mm in table 54.1. Just out of interest where does the 6mm come from?.

I believe the minimum size is 16mm buried without mechanical protection.

 

[telectrix]

my bad. missed the "buried". most read post fully before replying.:disguise:

 

[Smart Sparks]

Thanks all for your feedback, appreciate it.

 

[trebor]

By definition, AC current comprises of positive and negative half cycles with respect to a zero reference point, and an AC fault current can start to flow to earth at any point during either half cycle. An AC Type RCD will only trip in response to either the positive or negative half cycles of the AC earth fault current, whereas an A Type RCD will respond to both half cycles. As the AC Type RCD is blind to either the positive or negative half cycles of an AC fault current, the fault current could flow for up to 10mS before the RCD will see the fault current. This 10mS will be added to the response time of the device.
RCD testers are usually provided with a switch to enable the user to start the flow of the test current at 0 degrees or 180 degrees, i.e. starting on a positive going or a negative going half cycle respectively. By starting the testing of the RCD at both settings of the test current conduction angle, the user will be able to determine the maximum trip time of the RCD. This can be done at different test current levels.

 

[MarkieSparkie]

By definition, AC current comprises of positive and negative half cycles with respect to a zero reference point, and an AC fault current can start to flow to earth at any point during either half cycle. An AC Type RCD will only trip in response to either the positive or negative half cycles of the AC earth fault current, whereas an A Type RCD will respond to both half cycles. As the AC Type RCD is blind to either the positive or negative half cycles of an AC fault current, the fault current could flow for up to 10mS before the RCD will see the fault current. This 10mS will be added to the response time of the device.
RCD testers are usually provided with a switch to enable the user to start the flow of the test current at 0 degrees or 180 degrees, i.e. starting on a positive going or a negative going half cycle respectively. By starting the testing of the RCD at both settings of the test current conduction angle, the user will be able to determine the maximum trip time of the RCD. This can be done at different test current levels.

The above description of RCD types is in my view misleading and in part wrong.

[FONT=&amp] Semi-conductor devices are now incorporated in equipment used throughout industry, commerce and in the home. Typically, the purpose of these semiconductor devices is for monitoring and controlling industrial equipment e.g. speed controls for small motors and temperature controls, along with extensive use in computer equipment and household appliances e.t.c. As the equipment is fed from the mains electrical supply, in the event of an earth fault the presence of semi-conductors may result in the normal ac waveform being replaced by a non-sinusoidal fault current. In some cases the waveform may be rectified or chopped. These waveforms are said to contain a pulsating dc component which can either partially de-sensitise or totally disable a standard Type AC RCD.[/FONT] Because of this phenomenon Type A RCDs [FONT=&amp]were introduced, [/FONT]where tripping is assured[FONT=&amp] for residual sinusoidal alternating currents and residual pulsating direct currents, whether suddenly applied or slowly arising.[/FONT]

 

[telectrix]

The above description of RCD types is in my view misleading and in part wrong.

[FONT=&amp] Semi-conductor devices are now incorporated in equipment used throughout industry, commerce and in the home. Typically, the purpose of these semiconductor devices is for monitoring and controlling industrial equipment e.g. speed controls for small motors and temperature controls, along with extensive use in computer equipment and household appliances e.t.c. As the equipment is fed from the mains electrical supply, in the event of an earth fault the presence of semi-conductors may result in the normal ac waveform being replaced by a non-sinusoidal fault current. In some cases the waveform may be rectified or chopped. These waveforms are said to contain a pulsating dc component which can either partially de-sensitise or totally disable a standard Type AC RCD.[/FONT] Because of this phenomenon Type A RCDs [FONT=&amp]were introduced, [/FONT]where tripping is assured[FONT=&amp] for residual sinusoidal alternating currents and residual pulsating direct currents, whether suddenly applied or slowly arising.[/FONT]

in other words, a bit of electronic trickery.