Om Technical

Electrical Safety- NGR Monitoring System

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Introduction-Types of Power supply systems

Earth is a reference for all types of power supply and distribution systems. Proper selection of power supply and distribution system and protective measures depends on the criticality of the equipment, processes and reliability required. The issues to be considered are a) likelihood of insulation faults b) basic and fault protection c) availability of power supply d) technical & economic considerations e) practicability of implementation. Three main types of distribution systems: a) IT System – All active parts are insulated from earth or one point of supply is connected to earth through a sufficiently high impedance. The exposed non current carrying conductive parts of the electrical installation are earthed individually or collectively. IT systems are unearther, floating systems and used in critical systems which need availability and prevention from fire and shock hazard. b) TN System –One point of the supply is directly earthed.

The exposed non current carrying conductive parts of the installation are connected to that point by protective conductors. TN system are used in industry in general where safety during faults is needed as per regulations.

Solidly Grounded System (TN) Graphic of three common grounding methods High Resistance Grounded System Ungrounded System (IT)

c) HRG/LRG System – One point of the supply is earthed through a Grounding Resistor/Impedance. HRG/LRG systems use the benefit of both IT & TN systems and is used is systems requiring limiting fault currents to specified limits.

Indian and International Standards:

• Indian Electricity Rule Clause 116 mentions that in the interest of safety, the magnitude of the earth fault current shall be limited to 750mA in 550/1100 V system for underground mines and oil fields and 50A in 3.3KV/6.6KV and 100A in 11KV system in surface mines by installing a suitably designed restricted neutral of supply. Further the effectiveness and healthiness of the protecting system shall be continuously monitored to avoid unsafe condition. Further the over-voltages should be monitored to be within safe limits. • Canadian Standard CSA M421 for use of Electricity in Mines Clauses 4.5.6 & 4.5.7 mentions that the overvoltage in case of fault shall be limited to 100V or less by using suitable neutral grounding device and supply to be de-energized if fault voltage higher than this. Also, the continuity of grounding connection to be monitored. Further audible and visual alarm shall be provided for providing overvoltage and earth fault current.

NGR Monitoring Solutions for Electrical Safety:

Safety in Mines and Oil & Gas industry in general is of paramount importance and is strictly governed by International and National Standards and Regulations. Electric power supplies from utilities are taken at medium or high voltages and converted and distributed to various loads at the required voltages. Based on the requirements of Surface/Underground mines electric power supply and distribution system use solidly grounded, high resistance grounded or ungrounded power supplies at various low & medium voltages. Electric equipments are either fixed or portable or require to be shifted as excavation progresses. Further large machines like excavators, shovels and draglines for excavation & material transfer are used. Trailing Cables of various types and sizes are utilized for supplying motive

electric power to these machines. These cables also carry the protective earth conductor for safety requirements. Cables run through rough terrain and are subjected to harsh environment and physical damage. To minimize the risk of fire, explosion and shock, earth leakage currents and insulation resistance is to be continuously monitored. Alarm and protective measures to be installed whenever leakage currents and insulation resistance values are exceeded beyond prescribed limits as required by standards & regulations. The Indian Electricity Rule 116-1 for mines says “The earth fall current should not be more than 50 Amp in mines in any condition”. The suitable Neutral Grounding Resistance (NGR) should be installed between neutral point (star point) and the ground to restrict the earth fault current to 50 Amps. In Oil & Gas industry electrical installations are designed to limit the fault current within 750mA. In outdoor distribution station there are many possibilities of damage to the neutral grounding connections due to various environmental effects causing open circuit or burning of NGR. This is a very dangerous continuity i.e., totally floating type. This may also result in failure of earth-leakage protection operating through neutral CT as the leakage current may not be able to flow through open circuit of neutral grounding connection. To prevent this type of failure, Fail Safe Relays must be installed. The fail-safe relay will continuously monitor the continuity of NGR and neutral grounding connections and will trip the feeder circuit breaker as and when any open circuit occurs in neutral grounding connection or in NGR. The fail-safe relay will ensure fail safe operation of neutral grounding system.

Reasons for limiting the fault current

• Reduce burning and melting effects in faulted electric equipment, such as switchgear, transformers, cables and rotating machines. • Reduce mechanical stresses in circuits and apparatus carrying fault currents. • Reduce electric-shock hazards to personnel caused by stray ground fault currents in the ground return path. • Reduce arc blast or flash hazard to personnel who may have accidentally caused or who happen to be in close proximity to the fault current. • Reduce the momentary line-voltage dip occasioned by the occurrence and clearing of a ground fault. • Secure control of transient over voltages while at the same time avoiding the shutdown of a faulty circuit on the occurrence of the first ground fault.

Continuous monitoring of the NGR System with Bender NGRM500:

In 2017, Bender presented a new global benchmark for monitoring the grounding resistance (NGR) in HRG systems (high resistance grounded systems) with the NGRM700. This benchmark has since been successfully launched on the market and is being continually optimized. Bender expanded its portfolio even further in 2019 by an additional device for monitoring the grounding resistance: the NGRM500. The measuring technology of the NGRM500 is technologically just as powerful as the NGRM700. The NGRM500 series is capable of AC- and DC-sensitive measuring technology for the analysis and safe operation of the most modern types of loads in a resistance grounded system. The harmonics analysis of the residual currents with the associated freely configurable evaluations in the frequency range for the response values, which was first presented for the first time in the NGRM700, were also implemented in the NGRM500. They can be used for both notification and analysis purposes. In order to fulfil the plant operator's requirements, the Bender NGR monitors check the active and passive resistance as well as the current and voltage in the NGR. The limit values can be individually set according to customer-specific requirements and/or in accordance with regional regulations and specifications. This can be carried out as usual either directly on the device or via the standard web server included. The server provides faster access to the data as well as the possibility of carrying out initial analyses without the need for external software. All the data can be read via Modbus depending on the complexity of the plant, the available infrastructure, and the operator's preferences. www.omtechnicalsolutions.in

Application with NGRM500:

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NGRM500 Monitors:

• Residual Current in the grounding conductor • Voltage monitoring between transformer, neutral and ground • Monitoring of grounding resistor • Coupling Device from 1kV to 25kV • Communication • Modbus RTU • Webserver • Analog 4…20mA

Please note: The N connection of CD-series coupling device should be as close to the transformer star point as possible EM

Contributed by: Mr P. K. Bhattacharyya, Director, Bender India Private Limited, NaviMumbai And Mr. Timir Doshi, Managing Director, M/s Om Technical Solutions, Mumbai

Contact us for more information

OM TECHNICHAL SOLUTIONS, 308/309, Devraj Mall, Dahisar (East), Mumbai - 400068, India www.omtechnicalsolutions.com I om@omtechnicalsolutions.com I 022-28481518/19, +91-9324218050