FAQ ON EARTHING STANDARDS

 

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

  • Standalone RMU – maximum 10 ohms per electrode where 2 electrodes exist per RMU.
  • 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.
  • 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 can not 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.

 

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?

  • The 10ohm is a standard industry resistance that is required to allow protection to operate correctly
  • It has also been to ensure adequate surge protection operation
  • 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.
  • 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?

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

 
 

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 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

 

7. What are the Standard earth resistance values?

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