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Discuss Ze on earth rod in the Industrial Electrician Talk area at ElectriciansForums.net

S

sparkyork

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did some work for a nice old couple and when it came to doing the testing i was getting 308 ohms at the earth rod, i know it says less than 200 is better but wondering what the best thing to do as certificates go?!

cheers
 
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did some work for a nice old couple and when it came to doing the testing i was getting 308 ohms at the earth rod, i know it says less than 200 is better but wondering what the best thing to do as certificates go?!

cheers
Nip back after its rained and test it again........ shudda dropped under 200 then!

Never could get my head around TT systems:confused:

Mark

Look at you sparkyork a "respected member" now!

Mark
 
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S

sparkyork

  • Thread Starter Thread Starter
  • #3
yeah only cos im a posta holic!!

thing is i soaked the rod in the fist place to get it down to this reading so dont know if it will improve at all, can i not write something on the minor works cert??
 
H

heathelect

sparkyork,for tt systems look at table 41.5 page 50 in 17th edition regs, max zs for 30ma rcd operation for non delayed rcds is 1667 ohms, so your reading of 308 ohms should be ok, if anyone can say otherwise let me know.
regards dave

failing the above a bucket of water on rod works wonders
 
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T

tony.towa

If you want a better reading do not wait for rain or get a bucket of water, urinate (that's nice and polite) on it. The natural salts add to the conductivity.
 
G

Grae79

did some work for a nice old couple and when it came to doing the testing i was getting 308 ohms at the earth rod, i know it says less than 200 is better but wondering what the best thing to do as certificates go?!

cheers
record it, make a note about it on certificate. point out to old bods that it needs sorted, and why. :)
 
S

sparkyork

  • Thread Starter Thread Starter
  • #8
i did the job to the 16th tho and its an old board with 3871's and a 100ma time delayed main rccb
 
P

prideofengland

308 ohms after soaking it is a very high reading for a rod, the 1667 ohms refered to by heathelect is actually the max zs for an RCD circuit not the max reading for a earth electrode. The regs consider anything over 200 ohms as unstable, the NIC reduce this value to 100 ohms.

Therefore as far as certification is concerned this installation does not comply, Try a new position for the rod or add more to reduce the resistance.
 
S

Shakey

guys all this talk of soaking, urinating on, or waiting for the rain on rods is missing the point.

suppose you do relieve yourself over the rod and get below the made up figure of 200ohms, then woopdedoo.

Presumably the couple will give you a ring when they think there is a fault about to occur?

or maybe you could put an entry on the certs "customer to use own bodily fluids to reduce fault path on a regular basis"

it is the STABLE reading that you want, not an artificially low reading that you have created!

regarding the 1667 ohms, it is irrelevent whether or not you are using a spike or / and on a TT system. if 1667ohms or less is achieved the 30mA RCD is going to trip in the requireed time and before the touch voltage reaches 50V

Rich, TT's are not a problem. If you are concerned about the reading and want to sort it, go in deeper with the rod. We used to use rods with a threaded end, then put in an line copper ferrule on it and keep driving in. Alternatively you add additional spikes, and connect together. If you were using four spikes the accepted method is to connect them in a 'box' formation, this gives a better overall resistance reduction.

Just be aware, that the reduction is on a non linear scale, both for deeper rods and/or additional rods.

i.e. if a rod was 300 ohms for example, driving it in twice as deep, or providing a second rod would not half it to 150, you are more likely to get about 230 - 240 ohms.

and when i said 'made up' figure of 200 ohms, it is becasue the reading is just a 'supposed figure'. below 200 is likely to be more stable than above 200. but lets say it was 200 in the winter (when the readings are lower) and in the summer it was 400, as long as it stays below your 1667 ohms (presuming 30mA used) then you will still be ok.

even with a 100mA RCD you have 500 ohms to play with!

and remember the 1667 ohms is only to achieve 50V touch voltage, the 30mA RCD will still trip at values of up to 7667 ohms
 
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S

sparkyork

  • Thread Starter Thread Starter
  • #11
cheers shakey, i want to do things the right way and was concerned at this reading, as you say driving in deeper or doing a box section isnt gonna get me a nice low reading so in this instance and with you saying 100ma allow 500 ohms, then can i not assume therefore that all though reading is high its still gonna operate protective device? then make a note on cert??

also was on a job today thats just been rewired by some other copany, i was working on an existing immersion heater supply that i reused to feed a boiler in the loft. all protected by 30ma rcd and all main equip bonding ok. i measured R1+R2 and got 0.20 ohms, i then measured Zs at the same point and got 1.63 ohms, i then did Ze at the board and got 94 ohms (tt system!).

given that Zs = Ze + R1+R2 then : 94+0.20 = 94.2 ohms for Zs, yet measured Zs was 1.63 ohms, is this because the earthloop path is using the better earths available from the water and gas?
 
S

sparkyork

  • Thread Starter Thread Starter
  • #13
yeah thats what i was thinking so the electrical current is earthing through these rather than the earth rod??
 
W

wayne

probably , when im testing ,say a lighting circuit it may have other earth paths through circuits not disconnected or its attatched to steel .its not likely to lose these earths so the result stands
 
S

sparkyork

  • Thread Starter Thread Starter
  • #15
it kind of looks like Zs is a more definitive reflection of condition of the earthing then in this case. but the nic etc dont linke you to get r1+r2 by calculation either
 
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