Discuss Three phase earth 'rod' in the Electrical Forum area at ElectriciansForums.net

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imago

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I went to look at a job the other evening on a farm which had barns converted to storage and light industrial units. I was there to look at wiring up one of the units for socket outlets and lights. When I got there I was pleasantly surprised to see a brand new meter for each of the four units along with new SWA feeding a brand new hager DB in each.

So I measured up, and sorted out what they wanted where, added up the figures then went back for a closer look at the supply. Four feeds from the main DB all with new switching so isolation won't be an issue. Everything looked new and shiny including the 16mm earth cable, but then I noticed that it was curled into a pig tail and went behind the pallet I was standing on. I moved the pallet which turned out to be the lid for the earth pit. I say earth pit, I mean hole chisled into the concrete floor of the barn. I moved the pallet and found that the end of the cable was clamped to a 3/8" rod, and noticed that the top of the rod had been hacksawn off. I didn't get the meter out of the van, I'll leave that pleasure for a return visit.

If I get to go back I'll take a few pictures, but I'll have to put a warning in the title so that those of a squeamish nature don't look!

I couldn't help but wonder why anyone would have all that work and new kit, then stick in a screwfix special to try and get an earth. Never mind two metres from the building, smack a hole in the floor, that'll do. :crazy:
 

telectrix

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and noticed that the top of the rod had been hacksawn off.

typical bodgers. should have used an angle grinder.
 
K

Knobhead

If you can find any re-bar in the floor link to that to create an Uffer earth system. You may find yourself with good readings.
 
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imago

imago

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To be honest as I didn't stick the meter on it the readings may well be fine already, it just looked dog rough.

For the sake of a few minutes work and a couple of quid I don't know why the reinforcing and steel columns aren't used for earthing before the concrete's poured on these sort of buildings when they're put up. It'd be one addition to the building regs that'd make sense and be useful. Oh, wait, that's why.
 

snowhead

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If you can find any re-bar in the floor link to that to create an Uffer earth system. You may find yourself with good readings.
You wouldn't have said that before 08/10/2011.

I've just been reading that Rebar thread, I always thought they were called Rebar Earths.

I watched them do a telephone exchange earth in concrete once, there was miles of copper in it.
 
K

Knobhead

The first substation I was involved in building was back in 1972. That’s when I first had dealings with the system.
 
D

DurhamSparky

What's a 3 phase earth!!!??? Just though it was called an earth??

I might wind up some of my local DI now poising as a client needing a three phase earth!!!
 
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imago

imago

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OK, OK. :rolleyes2: You're quite right, it's an earth.

It made sense when I was posting it as I was refering to the earth from a three phase board. :)
 

electric will

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Talking about 120 degrees, is the whole electrical system, ie from the power station ran at 50 hertz and 120 phases 120 degrees apart, surely the turbine can't be rotating at 50 times a second or is there multiple windings in it, what I mean is the voltage generated many times a revolution, times 3 phases.
hard to explain but do you know what I'm getting at
 

Rob

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Talking about 120 degrees, is the whole electrical system, ie from the power station ran at 50 hertz and 120 phases 120 degrees apart, surely the turbine can't be rotating at 50 times a second or is there multiple windings in it, what I mean is the voltage generated many times a revolution, times 3 phases.
hard to explain but do you know what I'm getting at
The turbine will have a gear box...
 

johnboy6083

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The voltage produced at power stations is about 22000 volts, this is then stepped upto 400,000 volts by a transformer. It's then reduced to 132,000 volts near the destination. It is then stepped down again to 66,000 or 33, 000 volts. Very close to the customer it's reduced to the final voltage. This is often 400v for industrial customers, or 230v for domestic. Very large industrial premed use can be fed directly at much higher voltages.
 

electric will

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I know about the voltage stepped up and down was just wondering about the frequency, also I know there'll be a transmission of some type between the turbine and rotor, but that rotor turning in the magnetic field must determine the frequency,
when I first thought about it I was thinking the rotor must spin at 50 times a second which can't be possible, there must be multiple poles With in the stator
 

Rob

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It's only 3000rpm not really that fast.
 

electric will

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That's revs per min not per second, say it was 3000 revs per second (wouldn't want to be stood next to it) that'd determine the 50 hertz with 2 poles, there must be many poles so power is generated many times per revolution, allowing the rotor to spin slower.
 

electric will

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Why give up, generator with 2 poles spinning at 1 rev per second surly would produce a frequency of 1 hertz? So to get 50 hertz you could say itd need to spin at 50 revs per second-not really possible but with multiple poles the out put would be produced more times per revolution?
 

Rob

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electric will

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Yeah penny's dropped, cheers for being so helpful, I was nearly there one of you could have put me right in one sentence, most people would not bother to understand how things work but I like to know, don't see why other members on here have to be so bitchy.
 

Guitarist

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Yeah penny's dropped, cheers for being so helpful, I was nearly there one of you could have put me right in one sentence, most people would not bother to understand how things work but I like to know, don't see why other members on here have to be so bitchy.
I think it's great that you want to understand this stuff, as most electricians not from an industrial background have never even connected a motor, and they think "poles" are the guys trying to take their work.
This thread wasn't really bitchy at all tbh.... Now, if you'd asked what size MCB for 1.0mm cable, THEN you would have seen bitchy ;)
 

electric will

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I think it's great that you want to understand this stuff, as most electricians not from an industrial background have never even connected a motor, and they think "poles" are the guys trying to take their work.
This thread wasn't really bitchy at all tbh.... Now, if you'd asked what size MCB for 1.0mm cable, THEN you would have seen bitchy ;)
Whats a Mcb?
 
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