Earth System – Current Injection Testing (CIT) ServicesClick Here To Download Brochure
Depending on the size of the facility being tested, Liberty Consulting uses fit for purpose test equipment to accurately measure earth grid impedance, step, touch and transfer voltages, earth potential rise (EPR) contours, current splits in overhead earth wires and cable screens, and electrical continuity of earthed structures in high voltage substations, switch-yards and power plants. Testing of lightning protection earthing systems is also able to be performed accurately by experienced LCS engineers and suitable test equipment.
Accurate earth grid system testing
Large earthing systems can be difficult to test because they are often bonded to adjacent services and other earthing systems, and have very low impedance – well under one Ω, often as low as 0.1 Ω or below.
High power 50 Hz injection has been the “traditional” test method for transmission and distribution earth grid systems. However low current off-frequency injection has been used widely over the last 20 years, and provides reliable, accurate test results for large, extended earthing systems.
In particular, current injection testing (CIT) of earth systems allows identification of hazardous transferred voltages, which is a major concern for large substations in built up residential or industrial areas.
Small portable earth testers are designed mainly for small “stand-alone” earth systems and are not able to accurately measure low grid resistances (less than 0.5-1.0 Ω).
Fall of Potential (FOP) test method
The most prevalent and accepted method to measure the EPR and impedance of an earthing system is the “Fall of Potential Method”. This is done during the CIT.
This is not to be confused with the simplified three-point or 61.8 percent (straight line) earth test commonly recommended in portable earth tester manufacturers’ instructions.
The FOP test involves setting up a remote electrode system, a minimum distance away from the grid under test. A test current is injected into this remote grid location, either via a temporarily deployed test cable, or some fixed metallic connection (e.g. out of service overhead phase conductor). This will cause, on a smaller scale, an EPR able to be tested.
The remote current injection electrode (RCIE) at the end of the temporarily deployed test cable is established using a number of interconnected driven rods, often wetted down to achieve as low a resistance as possible.
Voltage measurements are then made at regular intervals from the grid under test, preferably at 90 degrees to the current test cable. Determining the EPR profile requires many voltage test points, and graphical analysis is required to detail the FOP response.
Test current, frequency and injection circuit
The test current should be high enough (1-10 A) so that voltage measurements are resolved with adequate accuracy. The test current level depends on the size of the grid under test.
The test current should be generated “off-frequency” close to the operating frequency of the power earth system (e.g. 55-58 Hz). A portable, hand-held tuned voltmeter (TVM) is used to measure voltages generated by the test-current induced EPR.
The curve below indicates what the FOP test response should look like.
What to measure
Often more than one voltage test traverse is required in order to obtain an accurate 360 degree EPR contour around the main grid, especially in built up residential areas.
As well as measuring grid impedance and surface voltage contours, step, touch and transfer voltages are tested using voltmeters tuned to the off-frequency test current. Test current splits among connected earth paths are able to be measured with Rogowski coils connected to the TVM.
Test voltages are scaled up to the actual Phase-Earth fault level, and represent actual touch voltages.
In addition integrity testing of earthing connections, and electrical continuity testing are also conducted.
Using CDEGS – MALZ to model FOP test results
The main earth grid and RCIE are modelled in MALZ using the tested soil resistivity profile. The test current is injected at 180 degrees into the RCIE, and at 0 degrees into the main grid.
A potential profile is then defined to coincide with the voltage test traverse(s), and the computed plot is compared to the measured curve for validity.
How often should testing/inspections be conducted
To ensure earthing systems are compliant with statutory and regulatory requirements, they should be tested when first installed, or when major modifications are made, and again every few years under a preventative maintenance program.
Sources of error during the CIT process
There are various sources of potential error during the testing process which need to be accounted for. The lower the grid impedance being tested, and the lower the soil resistivity separating the two test electrodes, the more pronounced these become:
Power frequency 50 Hz noise from the grid under test (if live). This is generally negated with the use of off-frequency test current, and tuned voltmeters to record data.
Mutual earth resistance of the grid under test and the RCIE, and the effects this has on the tested impedance value. The level of error is dependent on the distance between the two electrodes, and the soil resistivity. The further apart the test grid and RCIE, the better.
Mutual inductive effects between the current and voltage test leads. This is negated by the 90 degree separation between them, however practical site considerations may cause an off-90 degree angle, and this needs to be factored into measurements.
The voltage or current test leads being run parallel to conductive metallic components (such as fences, rail or pipe lines), which may carry a portion of the injection current.
Safety procedures during testing
The following procedures are followed by LCS testing engineers, and form part of our testing instruction which is submitted to the client for approval prior to any works commencing on site.
Work must be carried out under the control of a suitably trained person. All staff involved in the testing procedure shall be supervised and instructed to ensure that they do not touch leads or terminations during the test, except when instructed to do so. If there are other personnel on site not involved in the test, they must also be advised not to approach or interfere with the test leads. Particular attention needs to be given to supervision of the leads where they are laid across public, accessible areas.
Lightning or HV Switching
Work must not proceed during any lightning activity in the area adjacent to the substation or the power network connected to it. Work shall not take place if fault switching is taking place on associated networks. This requires testing engineers to contact the client’s representative to agree testing can take place.
Personal Protective Equipment (PPE)
The testing engineers and the people connecting/disconnecting the remote test electrode(s) must be wearing approved safety footwear, and appropriate PPE during the period of the test. This includes but is not limited to high visibility long sleeved shirt and long trousers, gloves, safety glasses, hard hat if required, and wide brim sun protection.
The route for running out both the current injection and voltage measurement test leads must be selected so as to minimise any risks. Wherever possible the route should not cross over roads or footpaths in frequent use, unless appropriate signs and barricades are in place. Where test leads are required to pass through fields containing livestock, they need to be continually supervised to avoid risk to the animals or damage to the leads. Test leads should not be run parallel with overhead power lines for any significant length.
The test supervising engineer must remain in constant communication with staff who are placing, connecting or disconnecting test leads. This is to be via approved two-way radios or mobile phones with speaker function.
Testing for voltages on earth system
Prior to any earth test, it is necessary to make sure that no significant stray voltages are present on the earth system under test. Some earth test instruments incorporate a voltage test facility which can be used, otherwise a separate tester is required.
Safe Work Method Statement (SWMS)
Prior to any earth test, it is necessary to have a prepared and approved safe work method statement identifying any perceived risks and the control measures required to eliminate or minimise these risks. All testing staff must have read and fully understood the SWMS and signed on accordingly.Click Here To Download Brochure