TT disconection time

[jamie-spark]

hi looking to see if you can buy new hager rcds to bs4293 s type as they have the 0.2 rather than the bs61008 0.3 disconnection time required for tt systems.

 

[Ponty Massive]

Not quite sure I follow here...Your Zs is going to determine your disconnection time. Provided it's within limit then it should disconnet in time. The RCD is used as a supplimentary disconnection unit and can and do fail.
0.2 / 0.3 seconds is 200 / 300 ms and most RCD will trip way under that time given a 30mA fault!

 

[jamie-spark]

hi sorry not made myself clear

i have a tt system . the main switch is a bs61008-1 100ma type s the ze is 75ohms and the zs is 4.75 the trip time is 357m/s when teting the main rcd but the regs say it should be 0.2 for a tt. is it ok ar am i confused.com

 

[IQ Electrical]

hi sorry not made myself clear

i have a tt system . the main switch is a bs61008-1 100ma type s the ze is 75ohms and the zs is 4.75 the trip time is 357m/s when teting the main rcd but the regs say it should be 0.2 for a tt. is it ok ar am i confused.com

Please check note 2 to table 41.1 you are missing the fact that this disconnection is far faults of negligible impedance, not 30mA.

Check the note and all will become clear.

 

[jamie-spark]

Please check note 2 to table 41.1 you are missing the fact that this disconnection is far faults of negligible impedance, not 30mA.

Check the note and all will become clear.

cheers IQ will do first thing regs are in the van

 

[IQ Electrical]

Them check appendix 3 for disconnection times at 2x I delta n for example and you can stick with the BS61008/61009 devices.

 

[topquark]

hi sorry not made myself clear

i have a tt system . the main switch is a bs61008-1 100ma type s the ze is 75ohms and the zs is 4.75 the trip time is 357m/s when teting the main rcd but the regs say it should be 0.2 for a tt. is it ok ar am i confused.com

I'm confused.com your Ze is 75 and yet your Zs is 4.75?

 

[telectrix]

but with a TT system now, under the 17th, you need a 30mA RCD, not 100mA. i suggest replacing the RCD with a 30mA 61008.

 

[Des 56]

hi sorry not made myself clear

the regs say it should be 0.2 for a tt. is it ok ar am i confused.com

411.3.2.4

 

[IQ Electrical]

I'm confused.com your Ze is 75 and yet your Zs is 4.75?

The 'Ze' or Ra as it should be termed will be without bonding conductors, the Zs at the DB will include all parallel paths and could well be as low as 4.75 Ohms.

 

[telectrix]

I'm confused.com your Ze is 75 and yet your Zs is 4.75?

seen bigger differences. Ze of 120 on TT, but Zs values coming down to under 2 ohms after connecting main bonding. obviously, the water supply pipework is a better earth than the TT rod.

 

[telectrix]

IQ you dedded me again. ( as bluebottle, the goons, circa 1956)

 

[topquark]

Have to say I'm a little surprised, that's one heck of a difference, doesn't say much for the rods. Guess I need to get out in the countryside a bit more

 

[boberty]

Had similar readings on a job recently. Took 4 goes to find a decent earth reading at 147 ohms for ze, then tested zs and with every thing wired in micc the reading was down to 4.75 ohms. The joys of tt systems.

 

[jamie-spark]

but with a TT system now, under the 17th, you need a 30mA RCD, not 100mA. i suggest replacing the RCD with a 30mA 61008.

Its for distribution circuits to steel containers all have consumer units with split boards. all out going circuits to 3 containers are done in swa. so the 100ma is ok