earth loop inpeadance,

Help Support Talk Electrician Forum:

greensparky

Junior Member
Joined
Aug 29, 2008
Messages
23
Reaction score
0
Hi guys I hope there is some one out there that can answer my question, I am learning to do electrical testing, and for practice I did a periodic inspection for a friend to see if it needs any work done before it is tested by an experianced tester, the test was on a mot station and mechanical work shop with overhead lines and is earthed on a TT system, I did an earth electrode test and got a reading of 6.61 ohms so with the R1+R2 measurments on each circuit the results are all over 7 ohms, this is where I start to get confused, because the regs say that a measurement of below 100 is OK for an RCD to opperate, Great but what about the circuit breakers these measurments are to high. so what is the presidence if you use an RCD does it not matter about the earth loop inpeadence measurment for the circuit breaker measurement ? I have looked through the regs and gn3 and another couple of books i bought but I can not find an answer does any one know where I can find out cheers.

 

seajayess

Senior Member
Joined
May 28, 2008
Messages
272
Reaction score
0
Dont know where you can find out about your question, but do know that if the earth loop impedence is to high then additional protection is required by the means of an RCD 30 mA. Remember that the earth loop impedence being to high will only efect the MCB when there is an earth fault, the MCB will still trip when there is a short circuit L/N fault. CJS :)

 
Joined
Mar 7, 2008
Messages
9,994
Reaction score
0
Location
In the office again.
CJS is correct. I`m wary of your 6 ohm reading for the rod; but we`ll ignore that for the moment.

The RCD which should be present on a TT (30mA) provides the ADS in the event of an earth fault. This will operate of an earth reading in excess of 1k5. (1k66 to be exact).

HTH

KME

 

greensparky

Junior Member
Joined
Aug 29, 2008
Messages
23
Reaction score
0
thanks guys for your input, it does not actuly have an earth rod it is using the structural steel that is concreted in the ground do you think that this should be replaced with and earth rod ?, what sort of resistance would you expect from a rod?, cheers.

 

ousme

Junior Member
Joined
Mar 26, 2008
Messages
5
Reaction score
0
You need to understand what you are doing.(TT system ,Ze, max Zs etc...)Do you have GN3 or the 17th edition one? Read and understand the Regs or you will be in trouble when someone get an electric shock from the job you certified.Have you done any electrical edition course?16th, 17th?

Regard.

Ousme

 

Mr Sworld

Electrical Technician
Joined
Mar 21, 2008
Messages
1,152
Reaction score
0
thanks guys for your input, it does not actuly have an earth rod it is using the structural steel that is concreted in the ground do you think that this should be replaced with and earth rod ?
Ohhh.... That's bad! :(

Are you sure there isn't an earth rod tucked away somewhere? The structural steelwork should be bonded, like water and gas, however it's not a recognised earthing point as far as I'm aware.

 

seajayess

Senior Member
Joined
May 28, 2008
Messages
272
Reaction score
0
Page 127 Red book Earth electrodes, Underground Structural Metalwork Embedded in foundations can be used as an electrode hope this helps CJS :^O

 

greensparky

Junior Member
Joined
Aug 29, 2008
Messages
23
Reaction score
0
Yes I Have Taken My 17th Regs And Am Awaiting My Results On My 2391, I Understand What I Am Doing But Would Just Like To Read In Black And White Where It Explains, Why Eli, Of Up To 100 ohms Is Ok For An Rcd But Not For An Mcb. I Have Looked In The Big Red Book And Gn3. But Can Not Find The Relationship, Can Any One Point Me In The Right Direction, Cheers.

 
Joined
Feb 27, 2008
Messages
11,034
Reaction score
0
With tt systems a line to earth fault the current that would flow would usually not be high enough to cause circuit breaker/ fuse to trip hence need for RCD to break circuit. It worries me as i have been to a few tt installations in recent years with standard split load boards with no Rcd protection on lighting circuits and any circuit on not protected side. I am suprised there are not more fires due to faulty electric's.

batty

 

steptoe

of course Im wrong, ask my wife™
Joined
Feb 13, 2008
Messages
24,337
Reaction score
117
Location
Gtr Manchester
1663 ohms is adequate earth for RCD protection, depends on your situation.

failing to understand this fundamental should make you realise then perhaps this is beyond your scope.

this is not meant to be condescending, simply a statement of fact.

 

Mr Sworld

Electrical Technician
Joined
Mar 21, 2008
Messages
1,152
Reaction score
0
Page 127 Red book Earth electrodes, Underground Structural Metalwork Embedded in foundations can be used as an electrode hope this helps CJS :^O
Well, that's me told! :^O

 

seajayess

Senior Member
Joined
May 28, 2008
Messages
272
Reaction score
0
the reason why you can use a RCD for additional protection over a MCB is that an RCD requires only 30 Milli amp differential between Line and Neutral to cause it to trip,( if it is a 30 m Amp RCD), where if your TT system has an Earth Loop Impedance of 100 ohms and a 6 Amp MCB ohms law, Volts divided by Resistance results in 2.3 Amps would flow to Earth and the MCB would not trip and any Exposed / Extraneous parts would be Live and that is with only a 6 Amp MCB hope this helps :)

 

ousme

Junior Member
Joined
Mar 26, 2008
Messages
5
Reaction score
0
Hi

Read Reg : 4115.5 to 411.5.4 ,table 41.5 note2 and 415 for RCD. Your breaker need to comply with Zs max.100 ohms is Max value proposed in a draft amendment of BS 7430,Code of Practice for Earthing.

Cheers

Ousme

 

seajayess

Senior Member
Joined
May 28, 2008
Messages
272
Reaction score
0
I think if its TT you can export the Earth you just need to install a stake/rod at the new CU as well CJS

 

steptoe

of course Im wrong, ask my wife™
Joined
Feb 13, 2008
Messages
24,337
Reaction score
117
Location
Gtr Manchester
Hi guys I hope there is some one out there that can answer my question, I am learning to do electrical testing, and for practice I did a periodic inspection for a friend to see if it needs any work done before it is tested by an experianced tester, the test was on a mot station and mechanical work shop with overhead lines and is earthed on a TT system, I did an earth electrode test and got a reading of 6.61 ohms so with the R1+R2 measurments on each circuit the results are all over 7 ohms, this is where I start to get confused, because the regs say that a measurement of below 100 is OK for an RCD to opperate, Great but what about the circuit breakers these measurments are to high. so what is the presidence if you use an RCD does it not matter about the earth loop inpeadence measurment for the circuit breaker measurement ? I have looked through the regs and gn3 and another couple of books i bought but I can not find an answer does any one know where I can find out cheers.
sorry, but am the only one that after hqving actually looked at this again reckons its a wind up???

NO way could anyone even think about this testings lack and be asking these questions??????

 

unphased

Senior Member
Joined
Feb 13, 2008
Messages
163
Reaction score
0
Location
Tamworth
1663 ohms is adequate earth for RCD protection, depends on your situation.failing to understand this fundamental should make you realise then perhaps this is beyond your scope.

this is not meant to be condescending, simply a statement of fact.
1663 ohms is NOT adequate for an earth rod. 100ohms is considered to be the most stable maximum, 200ohms is allowed. 1663 ohms is the value which satifies something else which you would be advised to read and check.(the 50V equation). 6 ohms quoted by the original poster on a TT system is very good! Don't know what all the fuss is about. However, the fusebaord should be of the type where either dual RCDs that discriminate have been used on a previous edition of the regs or appropriate RCDs are protecting appropriate circuits. 17th ed board will satisfy everthing in that respect.

 
Top