LV Earth Fault

Anyone know if the earth terminal is talking about 10 Ohm impedance, TN-S earrthing. How can we perform earth fault protection in Main switchboard?

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Please don't do that.

You call your supply network maintainer to get it fixed - 10 ohms is too high.
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Andrew Gabriel
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If the earth impedance is high then earth fault protection can be made using residual current device http://en.wikipedia.org/wiki/Residual-current_device
"A residual current device (RCD), or residual current circuit breaker (RCCB), is an electrical wiring device that disconnects a circuit whenever it detects that the flow of current is not balanced between the phase ("hot") conductor and the neutral conductor. Such an imbalance is sometimes caused by current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit"
"In the United States, the National Electrical Code, requires GFCI devices intended to protect people to interrupt the circuit if the leakage current exceeds a range of 4.6 mA of current (the exact trip setting can be chosen by the manufacturer of the device and is typically 5 mA) within 25 milliseconds."
"GFCI devices which protect equipment (not people) are allowed to trip as high as 30 mA of current. In Europe, the commonly used RCDs have trip currents of 10-300 mA."
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Tomi Engdahl (http://www.iki.fi/then /)
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I have a 10mA trip protected socket on my electronic test bench and a 30mA trip on the mains between the supply authority fuse and my consumer unit.
I have never seen an RCCD with a trip higher than 30mA on sale in the UK although earth leakage trips in industrial switchboards can be a lot higher than this.
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Stuart Winsor

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Usually I posted on news group, no reply is back. THANKS! The final circuits are protected by RCD. However, the main switchboard can't perform E/F protection. That is the problem, as if there is a earth fault in main riser (say busbar), the value is talking about below 50A. Main CB will not trip but a 50A flow continuous from one phase to earth as leakage>energy loss>thermal>voltage drop? Samuel Chan
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Well, as someone else has already pointed out, an impedence of 10ohms to earth is way too high and needs rectifying as a matter of extreme urgency.
However, as I understand it, earth fault protection does not depend on the quality of the earth at your switch-board, as an earth leakage won't neccessarily put current down the earth continuity conductor to your switch board anyway.
Earth leakage is measured by summing the currents in all your live conductors, with due regard to phase, and the answer should be zero. The neutral conductor should be regarded as a live conductor in this context.
I assume your switchboard has an earth-fault relay built into it and you can see what it is set at. It may also have an earth leakage meter. If you are worried, carefully [1] connect a load between a phase conductor and earth at your swithboard and see if either you get a reading on your meter or an E/F trip. If you have a meter, a light bulb should be sufficient to give you an indication.
This advice is given in good faith based on my understanding of the situation but I accept no resonsibilty for any harm, hurt or damage which may occur to persons or equipment or the consequences of such.
[1] That means with all due regard to your own and anyone else's safety.
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Stuart Winsor

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I worked for a project somewhere in the world and this area's Supply Regulator stated 10 Ohm. (That is the root of my question) I am also know that there is many possible ways for the leakage current to leak away... The Supply Regulation also stated that it is possible to provide some warning (aud./vis.) instead of protection. (I think this is their local experience on this matter). Anyway as an electrical engineer, I am trying to find any possible protection method (As I born/live/work in HK, I think that this is my stupid or something that I haven't met/learnt before...) Many thanks! Samuel Chan
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You may be mixing up two different things a ground rod, to infinite earth ground resistance is often specified at 10 ohms or less. THat is a completely separate issue with the bonding resistance, which is intended to be as close to 0 as possible. jk
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In that case, you need to put some relaying on your neutral leg of the incoming power supply and do something around Zero Sequence protection.
H.

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At the very beginning of this matter, I've thought about zero squ. relay. But a moment later, I turned down it. As 1st, I do not have experience on this, 2nd it is a long time & hard to get it, 3rd as discussed before, it is possible that the leakage current leak away in some other paths (may be I'm wrong)? The likely solution is No E/F relay (just O/C relay) w/ E/F detector that pass out the warning signal. Many thanks. Samuel Chan
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That is OK as long as you realize that you cannot count on it working with the same degree of certainty as if you had relaying on your neutral leg.
H

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10, 20 and 30mA ratings are used for personell or livestock protection, there are also higher rated units of 100,300 and 500 mA units available for the domestic scene - these are installed at the main incoming point and have a time delay, to give discrimination with e.g. 30mA downstream. These units provide fire and property protection - not personnel protection.
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Thanks to all. Summary: MG also have leakage device (E/F or leakage) for their Circuit Breaker. The 10 Ohm requirements on Abu Dhabi seem allowed NO leakage on main BUT required on sub-main. The site condition seem not as bad as the Electricity Rules required, likely a low earth pit can be formed.
BUT I were terminated w/ NO reason after above case.
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