TT system - Why less than 200 π ?

[HappyHippyDad]

I have had a read through a few threads on a TT system needing to be below 200Ω and I wondered why the regulations state this figure?

I dont mean why cant we just let it be higher (as it will be protected by an RCD - so in theory (not good practice) just needs to be below 1667Ω)), I mean why 200? Where does this figure come from? Who decided on this figure and more importantly why? What happens at 200Ω that makes it safer than 250 or 300 for example?

Why not have a much lower figure that might actually aid in disconnection times. Is it to do with the logistics of having to add maybe 3 or 4 rods to achieve such figures or other reasons?

Thanks all...

 

[HappyHippyDad]

Learn something new every day!! lol!!

Now has anyone here ever seen one of these earth leakage breakers from the middle to late 1930's because i know that i haven't?? So i'm wondering just how generally available they would have been at that time? Not very would be my guess, in fact probably as rare as hens teeth!!

Whether they were included in the IEE reg's of the time, i wouldn't like to say yea or nea now! ...lol!!

Starting work in the 70's for a very large manufacturing company on a site that covered an area of a couple of square miles with 7 factories including a power station/forge and a foundry, the only earth leakage devices i knew of, was limited to main switchboards and main distribution boards in the way of protection relays. Only in the latter part of my time at the company, did i see RCD devices being brought into service, that would be around the same time as circuit breakers and MCB's etc were replacing fuse board installations....

Found a picture for you E54 Old GEC 'ELF' 30 amp earth leakage circuit breaker

 

[Knobhead]

The first ones I fitted at work were Hackbridge Faraday 30mA. Talk about belt and braces, we fitted them to earth proving units.

My first house I fitted a couple of up front Crabtree 500mA units. Colleagues thought I’d lost my marbles as they weren’t required under the 14[SUP]th[/SUP]. They were bloody expensive at £65 each (£461 in today’s money).

 

[Bobster]

Any advance on pages 401 → 412

PS,
Finally found out where the figure of 1/10,000[SUP]th[/SUP] for maximum leakage current came from. I was taught it as an apprentice and have used it ever since for power systems.

It's also mentioned in the M&Q regs.

 

[Knobhead]

This is the first time I’ve found it outside M&Q.

Could you imagine the chaos had we applied the blanket 1MΩ. Half the motor circuits would fail.

For those not in the know, testing a motor you can’t work to 1MΩ.

Minimum IR for a 400HP 550V motor would be 14KΩ.
We would also apply this to power circuits where older cables are in use.

 

[shanky887614]

This is the first time I’ve found it outside M&Q.

Could you imagine the chaos had we applied the blanket 1MΩ. Half the motor circuits would fail.

For those not in the know, testing a motor you can’t work to 1MΩ.

Minimum IR for a 400HP 550V motor would be 14KΩ.
We would also apply this to power circuits where older cables are in use.

is that 14MΩ across the windings?

 

[Bobster]

To earth

 

[Marvo]

is that 14MΩ across the windings?

that's 14kΩ not 14MΩ and I think he's talking about IR to earth. We've installed equipment on mines where there's 40Amps earth current being lost to earth at the main DB and it's just an accumulation of acceptable earth currents, not considered faults.

 

[Knobhead]

To earth

The value is the same between Ph→E and Ph→Ph although the voltage is different.

You have to calculate the individual winding current which happens to be √3 of FLC and the funny thing is Ph→E voltage is √3 of Ph→Ph.
So one value for both.

But I would test Ph→E @500V and Ph→Ph @1000V

 

[Bobster]

1/10000 would be righ at 500v across phases.

I, like you was taught to test at 1000v on larger motors. I can't remember what we used without looking over books, but we were allowed less than 1/10000. It wasn't a liner amount though.

 

[moospark]

To earth

Ashes to ashes,

 

[moospark]

We know Major Tom's a junkie

 

[DPG]

We know Major Tom's a sparky. Daz

 

[moospark]

We know Major Tom's a sparky. Daz

Even better.

 

[Knobhead]

1/10000 would be righ at 500v across phases.

I, like you was taught to test at 1000v on larger motors. I can't remember what we used without looking over books, but we were allowed less than 1/10000. It wasn't a liner amount though.

The usual line would be “it’ll be OK once it’s run and warmed up a bit.”

Faced with the choice of mauling my ******** off changing a motor or press the button to see what happens, you don’t need two guesses which won. If it goes bang it needed changing anyway.

I’ll be honest RoB, I’ve let them go at ridiculously low readings. If it blows up who’s going to know? There’s no point in the inquisition testing the motor to prove I was wrong after the event.