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Discuss Ze reading for TT system in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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jonboy1

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I have recently installed an earth spike within a garage and the Zs reading I got was 66 ohms and pf 3KA What is the permitted reading for a TT system within a garage?
 
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Guest123

  • Thread Starter Thread Starter
  • #2
Hey there.

A Ze (RA) reading of 66 ohms on a TT system is absolutely fine.

The accepted maximum is 200 ohms.

The maximum according to BS7671 is 1667 ohms, but a reading this high should never be accepted.

Cheers
 
J

jonboy1

  • Thread Starter Thread Starter
  • #3
Thats a relief. We were going to throw a bucket of water over the rod to get a better reading. He he!
 
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rob1967

  • Thread Starter Thread Starter
  • #4
Hey there.

A Ze (RA) reading of 66 ohms on a TT system is absolutely fine.

The accepted maximum is 200 ohms.

The maximum according to BS7671 is 1667 ohms, but a reading this high should never be accepted.

Cheers
If it say's a reading of 1667 ohms is the maximum according to BS7671 then why is the accepted value only 200 ohms? but even this is classed as unstable........
 

scotsparky

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Arms
over 200 is classed as unstable as the reading may change with weather conditions. For instance if the spike is put in in winter the soil is wet but come mid summer after sunshine (not in scotland) the soil has dried and the resistance has risen.

If the reading is under 200 the liklyhood of the summer reading being over 1667 is low
 
S

sharpy1983

  • Thread Starter Thread Starter
  • #6
great answer spotsparky. couldnt have put that explanation better myself
 

TaffyDuck

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Arms
I have recently installed an earth spike within a garage and the Zs reading I got was 66 ohms and pf 3KA What is the permitted reading for a TT system within a garage?
66 ohms and a pfc of 3000 amps, think something is wrong there mate.
 

telectrix

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Mentor
Arms
Esteemed
nah. he's quoted the pscc, not the pefc. still looks high though.
 

Marvo

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Mentor
Arms
Esteemed
Thats a relief. We were going to throw a bucket of water over the rod to get a better reading. He he!
We often use a hose pipe on the rod for up to 30 minutes. A thorough soaking of the area can help the ground to settle around the rod especially if multiple rods and threaded couplings were used which increase the diameter of the hole larger than the actual rod.

We would only soak once we had a decent impedance reading. Any test readings after soaking wouldn't be considered stable.
 
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E

Engineer54

  • Thread Starter Thread Starter
  • #12
over 200 is classed as unstable as the reading may change with weather conditions. For instance if the spike is put in in winter the soil is wet but come mid summer after sunshine (not in scotland) the soil has dried and the resistance has risen.

If the reading is under 200 the liklyhood of the summer reading being over 1667 is low

Stability of an earth rod is NOT, and should never be based on the Ra value of the earth rod. This is another misleading statement made in BS7671 you could have a perfectly stable 200+ Ra value. Stability as well as Ra levels are far more directly related to the driven or laid depth of the earth electrode, be they rod, tape, plate etc....

Don't think i need to mention what i think of the 200 ohm Ra value!! lol!!!
 

rich.250

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Arms
They recommend 200ohms as then it covers there arse, to operate most common rcds without a touch voltage exceeding 50 volts. 30-100mA

1667 is generated by 50v(touch voltage) divided by 0.03 (30mA)

The best way to see if your reading is good is just do the calculation for your selected rcd.

If it is too high more than likely you rcd tester will just say >50v touch voltage
 

rich.250

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Arms
Also need to keep in mind that
For additional protection x5 rcd tests on 30mA. So 150mA

50 divided by 0.15 = 333.33 ohms
 
E

Engineer54

  • Thread Starter Thread Starter
  • #15
Also need to keep in mind that
For additional protection x5 rcd tests on 30mA. So 150mA

50 divided by 0.15 = 333.33 ohms

RCD protection on TT systems are NOT classed as additional protection, they are the ONLY protection for earth faults!! That magic 200ohms max will be doing absolutely nothing!! lol!! Considering that the present RCD devices are not the most reliable of protective units, on UK TT systems, i would be backing any 30mA set-up with an up front 100mA S type RCD. At least it will give you a pretty decent level of additional protection if a 30mA device fails on you....
 

rich.250

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Arms
Hi e54, sorry I meant you will more than likely need additional protection in most installs now. Buried cables etc
I wasn't very clear lol

Yes 100mA s type upfront is what I do :)
 

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