Ground Finder Calibration and Testing

Good grounding is more than a safety measure—it not only prevents damage to railway equipment but ensures the safety of passengers and railway personnel working on tracks. National Electric Code (NEC) defines grounding as “a conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth or to somebody that serves in place of the earth.”

Studies reveal that electrical faults and improper grounding are two prime causes of downtime and revenue loss in the rail industry. These faults are responsible for the costs associated with litigation, loss of property, personal and corporate productivity. A railway earthing shall include the design of the entire earth-termination system for all structural components of the railway infrastructure to protect people in the railway environment and ensure reliable rail traffic.

Railroad Earth Grounding – Types and Operation:

The concept of Earth Grounding has two subjects:

1. System Grounding: System grounding is the intentional connection from a circuit conductor, usually the neutral, to a ground electrode placed in the earth. System grounding helps detect and clear ground faults. System grounding is to control the voltage to the ground within predictable limits.
2. Equipment Grounding: Equipment grounding ensures that the operating equipment within a structure is properly grounded. It provides a return path for ground-fault current.

These two grounding systems are distinct and shall be kept separate, except for a connection between the two systems. This avoids differences in voltage potential from a possible flashover from lightning strikes. Besides system and equipment grounding, two additional earthing systems are used for railway and traction systems – Mobile traction system earthing and Securing rail operations through earthing.

1. Mobile traction system earthing ensures the safety of personnel working in the track area by negates threats from the contact line (15000 volts). In an emergency, accident, or maintenance, all parts of the contact line system must be earthed through a connection with the running rail (the return wire). Merely switching off the current in the control room will not suffice.
2. Stationary traction system earthing encompasses the earthing design as a whole for all structural components of the rail infrastructure. The objective is the protection of people within the rail environment and the safeguarding of reliable rail operations. It is necessary to prevent voltage from being transmitted to people on the railway platforms or equipment in the event of damage, e.g., the severing of a contact wire.

Applicable Standards for Railroad grounding:

The following standards, codes, and guidelines are to be complied with when designing and implementing the grounding of communications equipment, including but not necessarily limited to:

1. Institute of Electrical & Electronics Engineers (IEEE) 1100: “Recommended Practice for Power Grounding Sensitive Electronic Equipment.”
2. Institute of Electrical & Electronics Engineers (IEEE) C62.41-2000: “IEEE Recommended Practice for Surge Voltages in Low-Voltage AC Power Circuits.”
3. ANSI-J-STD-607-A “Commercial Building Grounding (Earthing) and Bonding Requirements for Telecommunications.”
4. Institute of Electrical and Electronics Engineers (IEEE) 80: “Guide for Safety in AC Substation Grounding.”
5. Institute of Electrical & Electronics Engineers (IEEE) 81: “Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System”.

Ground Finders -To Test Ground Potential and Resistance:

Ground finders are digital multimeters widely used by railway personnel to detect ground resistance and standardize the testing for grounds required by FRA (Federal Railroad Administration, Department Of Transportation) Rules No. 234.249 and 236.107.

Why is Ground Testing needed?

Over time, due to percolation of rainwater, high salinity or corrosive soils, and raised temperatures can degrade ground rods and their connections. Despite the low earth ground resistance values during initial installation, these values can increase if the ground rods are eaten away due to corrosion.

Like the S&C’s Model 360 Ground Finder, Grounding testers are indispensable railroad ground troubleshooting tools to maintain uptime. Often, frustrating and intermittent electrical problems could be related to poor grounding or poor power quality.

All grounds and ground connections of railway applications must be checked at least annually as a part of a preventative maintenance plan. While performing the checks, a 20% increase in resistance should be checked. If the rise in resistance is identified, the rods should be replaced, or additional ground rods should be added to the system.

Ground Finder – Measurement Procedure:

Step 1- TEST mode: The device alarms (Red LED, beeper) activate when the maintainer encounters aground. For safety and to allow testing under traffic, the output current is limited to less than 1.0mA in the “TEST” mode.

Step 2 – CONFIRM Mode: Upon ground indication, the maintainer must have track and time (TOP) clearance to switch the tester to the “CONFIRM” mode before verifying that his ground is reportable.

Step 3 – POLARITY Test: Once he has determined that it is safe to switch the Model 360, a quick test in the “CONFIRM” mode tells him the ground value up to 10.0mA (meter full-scale).

Note:

• It is unnecessary to switch polarities to detect both positive and negative grounds if the circuit voltage is 16VDC or less. For circuit voltages above 16VDC, it will be necessary to make both polarity tests. When switching polarities is necessary, a simple push-button feature is supplied to enable a quick, temporary reversal of polarities.
• Ground finders are often designed with circuitry that gives protection against damage caused by momentary contact with 110VAC and a connection of the tester to a high-current ground.

Ground Finder Calibration – Why is it essential:

Ground finders are similar to a milliohm meter; however, these are capable of testing higher resistances, as required for railway applications. Calibrating ground finders at regular intervals is essential for the following reasons:

1. Federal Requirements: Title 49 of the CFR governs rail transportation. It provides standards for compliance with the Americans with Disabilities Act and with the safety of rail transportation. To comply with CDR norms, accurate ground resistance readings are required to maintain less than 5.0 ohms using a Ground finder. Inaccurate readings of the ground finder shall result in penalties.
2. Operational faults: Ground finders are sensitive equipment and shall require qualified personnel to measure gradually through “Test,” “Confirm,” and “Polarity” modes. Not following the procedures can damage the circuits.
3. Damage with High voltage contacts: Though Ground finders inherently withstand AC voltages up to 110 VAC, contacting with higher voltage can induce inaccuracies in the readings.
4. Ground finders such as Model 360 from S&S can test only DC voltages. Contact with AC ground potentials can damage the equipment.

Though Ground resistance measurements using ground finders seem straightforward, the equipment needs to be calibrated and maintained following OEM SOPs. To comply with international standards, the repeatability and accuracy of ground finders should be maintained by following an annual calibration process.

Calibration of ground finders also determines the calibration uncertainty value and the deviation from the actual reading. These readings are used as correction factors or to decide if the ground

e2b calibration offers industry-leading ISO-certified Ground finder calibration services. Our labs are ISO/IEC 17025 accredited and operated by a team of qualified calibration experts to test and calibrate your rail meters. Our verifiable services are unmatched in the industry. We are registered with ANAB. We are also ANSI/NCSL Z540-1-1994 certified. We have the NIST Traceable Wide scope of ISO/IEC 17025 accreditation. Contact e2b calibration for all your equipment calibration needs.