earthing resistance at jetty

1. Attach a single-core cable with large cross-sectional area to the star-point of the local distribution transformer, which will be earthed.
2. Run the cable out to the "jetty place", or wherever.
3. Measure ohms between the end of the cable and the item the earth resistance of which is required.
4. Record the value of the resistance.
5. Measure the end-to-end resistance of the single core cable.
6. Subtract one from the other
7. "Robert is your mother's brother".

1) What minimum readings do we have to get for:

a) Standalone RMU – maximum 10 ohms per electrode where 2 electrodes exist per
RMU.

b) Standalone TX (up to 500kVA and over 500kVA) - maximum 10ohms per electrode
where 2 electrodes exist per TX. When HV and LV earths are combined then final
installation shall be no greater than 1ohm.

c) Combined TX/RMU – maximum 10 ohms per electrode where 2 electrodes exist per
TX/RMU site. When HV and LV earths are combined the final installation shall be no
greater than 1 ohm.

2) Do we drill additional rods at Pillars to get to the minimum combined earth reading
of <1ohm?

Yes, this can be done only if <1ohm combined earthing cannot be met. This does not
eliminate the requirement of achieving maximum 10ohm at each electrode of a TX or RMU
site. It may be necessary to drill at pillars where the LV network (of new subdivision) is
sparse i.e. less than 20 pillars. Refer also to Q6.

Where additional rods are drilled at a pillar to achieve <1ohm combined earthing, the pillar
shall be marked "DEEP EARTH". The label will be fitted inside the pillar, be non
conductive and be indelible. The deep earth will then form an integral part of the new
installation and relocation of that pillar needs to account for the deep earth. The Deep
Earth shall be marked on the 'As Constructed' drawing. A deep earth depth is typically
30m.

3) Can we install counterpoise earthing, i.e. install an earth cable (bare or insulated)
between the transformer site and another electrode further away from the site?

No. The reason for this is earth potential rise can transfer voltages into adjacent
equipment and/or services. Our preference is to use the cable screens (neutrals) that are
insulated and connect them to deep earths at pillars.

4) Where does the minimum 10ohms per electrode come from?

a) The 10ohm is a standard industry resistance that is required to allow protection to
operate correctly.
b) It has also been to ensure adequate surge protection operation
c) Each electrode needs to be 10 ohm such that if the connection from 1 electrode is
broken (eg. for testing) or has been damaged the TX/RMU is still connected via the
other 10ohm earth electrode, i.e. redundancy required as per AS3000, Sec 7.8.10.3.
d) It is the level required (AS3000, Sec 7.8.10.5.2) for LV earthing for TX >500kVA when
LV earths are separated from HV earths. Reasons are similar to (a).

5) How many connections are required from the terminal bar and the earth electrodes?

Refer to the drawings showing on next page. The Terminal Bar (defined in AS3000, Sec
7.8.10.4.4) of the transformer consists of 2 parts that are interconnected i.e. LV bar and
HV bar. The reason for having an interconnection (Combined HV & LV link is factory
fitted earth cable) is to allow for separate HV and LV earthing if deemed absolutely
necessary. With a single Terminal Bar this would not be possible.

There are 2 connections to the earth electrodes, which also couple as a grading ring.

Electrode 1 (max.
10 ohms)
Electrode 2 (max.
10 ohms)


EQUIPMENT
ITEM

CMEN Areas

Separately Earthed Areas

Local Earth
Requirements

Area
CMEN

HV Earth
Requirement

LV Earth Requirement

Area
MEN

Standard
Pole-Mounted
Distribution
Transformer

Connect all
items to 30O
max local earth
and area
CMEN.

1O

max

Separate Earth system
not usually employed.
Requires approval from
Earthing Engineer prior
to use.

Separate Earth system not
usually employed. Requires
approval from Earthing
Engineer prior to use.

N/A

Ground
Mounted URD
Substation

Connect all
items to 10O
max local earth
and area
CMEN.

1O

max

Separate Earth system
not usually employed.
Requires approval from
Earthing Engineer prior
to use.

Separate Earth system not
usually employed. Requires
approval from Earthing
Engineer prior to use.

N/A

HV Switch
Poles
Country

Connect all
items to 10O
max local
earth and area
CMEN.

1O

max

Connect all items to
10O max local earth

N/A

N/A

- Less than 10 ohms on each earth electrode (rod)
- Less than 10 ohms combined on a stand alone RMU site
- Less than 1 ohm combined including neutrals on a TX site
- Less than 1 ohm combined including neutrals on a RMU & TX site.

6) What are the steps and process to achieve earthing for a TX that complies with the
standards?

There are two options for achieving electrode and system earthing resistance
requirements. One option is to install the earth equipment on site until the requirements
are achieved. The process for this is given in the table below. The other option is to
engage an Earthing Consultant before the subdivision is installed to undertake an earthing
design to determine the extent of earthing works required for the site that satisfies safe
touch, step and transfer potential criteria.

For both cases field measurements must be made to prove the network impedances.

Note: the earthing requirement for combination RMU and TX site is the same as a stand-
alone TX site.

Step

Description

Responsibility

1

Install (hammer or drill) earth rods at diagonal corners of TX site. Each
electrode must have a maximum resistance of 10 ohms.

Connect up all earths on new network , this includes transformer HV
and LV earths and pillar earths and check combined earthing resistance.
The aim of the test is to achieve 1 ohm or less resistance.

It has been found that in most cases a target resistance of 2.5 ohms is
aimed for per transformer electrode such that when connected together
and tested, results in <1 ohm.

Has <1ohm combined earthing been achieved?

Yes – finished.

No – go to next step

Developer/
installer

2

Drill deep earth

a) at pillar furthest from the TX site.
b) Check combined earth resistance does not exceed 1 ohm. If
exceeded proceed to step 3

Note: Drilling an earth at the furthest pillar is to ensure the pillar
earth is not within the zone of influence of the TX earth. Earths can
be installed within the zone of influence however they will not be as
effective in reducing the overall earth mat resistance.

Has <1ohm combined earthing been achieved?

Yes – finished.

No – go to next step .

Developer/
installer

3

Combined earth exceeded for single remote drilled earth

a) Drill at least 3 remote (2 additional) pillars from the transformer
and each other to try to achieve the 1 ohms combined earthing
resistance
b) Where the 1 ohm combined earthing resistance is not
achievable the maximum combined earth resistance permitted
is 3 ohms before going to step 4

Has <1ohm combined earthing been achieved?

Yes – finished.

No – go to next step

Developer/
installer

4

Interconnect the new subdivision LV neutrals with the existing adjacent
network LV neutrals.

Has <1ohm combined earthing been achieved?

Yes – finished.

No – go to next step.

Developer/
installer

5

Suitably qualified engineer to conduct touch, step and transfer potential
calculations to determine acceptability of installation.

Note, separate earthing will only be considered when all the above has
not achieved the desired EPR.

Developer
/Earthing
Consultant

6.

Conform calculations

Western Power

7.

Copy of results and calculations must be forwarded to WP.

Developer
/Earthing
Consultant


Probe C Probe P

7) What details do I need to submit to the Earthing Engineer, if at step 5?

Details to include:

a) Design Drawing of the site
b) Location, depth and earth resistance of electrodes installed (individually)
c) Combined earth reading with HV and LV connected (i.e. electrodes, grading ring,
HV and LV earth link all connected)
d) Length of test leads used to measure the earth readings (i.e. length to P and C
probe from the earth electrode)
e) Indication of the first HV protection device upstream from the site. If TX only site,
then the upstream fuse location and details. If RMU or RMU/TX site, then the first
upstream recloser or feeder circuit breaker.

8) What length of earth test lead should be used to measure the electrode
resistance?

Most testers adopt the Fall-of-Potential method using a Current probe (C) and a Potential
probe (P) that is placed 62% the distance of probe C from the earth electrode under test.

When earth testing with Megger, if depth of the electrode is I, then the test probe P of the
earth tester must be placed at minimum (or further) 2xI from the earth electrode and the
C probe must be 3.2xI from the earth electrode (in that ratio where P probe is 62% the
distance between the earth electrode and C probe). This ensures an accurate earth
resistance measurement, otherwise the measurement will result in higher resistance
readings due to summation of the zones of influence of the earth electrode and the C
probe. Refer to arrangement below.

EXAMPLE - if rods have been drilled to 10m (I) then probe P must be at a minimum (or
more) 20m (2 x I) from earth electrode and probe C must be 32m (3.2 x I) from earth
electrode.


Below is a guide of test lead lengths required to test electrodes of various depths, which
should be followed to achieve correct earth resistance readings. The test lead length is
not limited to 320m, where the test equipment is capable of longer lengths.

ELECTRODE DEPTH

Test Lead lengths from Earth Electrode

Potential Probe (P)

Current Probe (C)

<15m

30m

50m

15 - 30m

60m

100m

30 - 45m

90m

150m

45 - 60m

120m

190m

60 - 75m

150m

240m

75 - 100m

200m

320m

9) Are the earthing standards changing and how will this impact construction?

Yes, the standards are changing such that AS3000:2000 is being revised and could be
issued as AS3000:2007 (release date is uncertain at this stage). The section on HV
earthing has been retained at present but will be superseded by AS2067, which is also
being revised at present. All HV earthing standards will reside in the new version of
AS2067, which does not define resistance values to be achieved but instead defines
touch voltage limits that are applied to a risk assessment as shown in the flowchart. This
means earthing systems need to be designed to meet the specific risk criteria for a
particular site and depends on:

a) Earth fault current and duration
b) Soil resistivity
c) Level of LV interconnection
d) Other factors…

It is envisaged that each design drawing will have on it a resistance value that needs to
be achieved and any other remedial earthing work required to ensure touch voltage limits
are satisfied, all of which are derived by undertaking a level of earthing design.

Standard resistance values of 30ohm, 10ohm and 1ohm may not exist.

10) Do transformers need to have MEN connection if one is provided in the LV
switchboard?

Yes, the transformer LV side must have a connection from LV neutral (either from neutral
bushing or neutral bar) to the LV earth bar on the transformer. Similarly, an MEN is
required in the Western Power LV distribution board which feeds the street or customers.
The customer also needs to install an MEN at their installation in accordance with
AS/NZ3000:2007. An MEN is required in the transformer because a separate earth cable
is not connected between transformer earth bar and LV distribution board earth bar.

MEN
Connection

MEN

Link


Determine tolerable bodycurrent limits refer toIEC 60479-1 figure 14Heart current factor fortouch voltage dependingon body pathIEC 60479Table 5Allowable body currentCalculate voltage acrossbare body contact pointsBody impedance atpower frequency seeIEC 60479Addbare hand toearthingpath resistance and ifapplicable individualprotection equipmentAddbare foot toearthingresistance and ifapplicable individualprotection equipment= Total path impedance

How abstruse.

Not only that, it is hard to understand comprehend. And he didn't even mention his uncle Bob.