• The metallic NGRs are designed for specific resistance values and voltages specially for shorter time ratings. The Metallic NGRs have the following advantages over a liquid type: • Their resistance value is precise and determined at the manufacturing stage remaining constant throughout their life. • Their temperature Time constant is shorter allowing a resistor of shorter time rating to be used. • They are easily to install on site.
• They require virtually no maintenance and have a longer useful life. • No anti frost heating is required nor anti condensation heaters generally. • Because of problem resulting from the evaporation of gel and electrolyte the liquid resistor require constant maintenance. • Invariably the type of cooling will be natural air-cooling. Neutral Grounding Resistors
Neutral Grounding / Earthing Resistors are employed in AC Distribution Networks to limit the Restricted Earth Current to a safe value which would flow from the Transformer or Generator star point in the event of an earth fault in the system without exceeding specified temperature limits. In case of short time rating, dissipation is negligible & the temperature rise for the resistor element depends on its capacity to store the energy generated in proportion to the mass & specific heat of the material used. Theses NGR’S are connected between the neutral point of transformer / generator & the earth. • In no Earth fault condition continious current of 5 % to 10 % of full load current may pass through the resistor due to unbalance in system. • In Earth fault condition the Restricted earth fault current passes through the Metallic Resistor protecting the system till automatic protection takes place by absorbing the earth pressure, protecting the winding etc. When the current flows continuously for long duration, the temperature of resistor element increases and then stabilizes at a value for which energy supplied is entirely dissipated to the atmosphere. Hence these resistors are designed for allowing maximum heat dissipation
Important Definitions & Construction:
• Rated Voltage : For neutral grounding / earthing resistor rated voltage means line- to neutral voltage i.e. System Voltage divided by root3 i.e. 1.732. • Rated Current : Means Initial current that will flow through resistor when it is cold. Normally Full load current value is same as Rated current. • Time Rating : It is length of time for which NGR must tolerate rated voltage. • Short Time Rating : Normally it is 10 sec. or 30 sec. or 60 sec. depending on design parameters of protection system. IS-3043 recommends 30 sec. rating. • Continuous Rating : It is normally 10 % of full load current for healthy system neutral grounding resistor to be designed for continuous rating of 5 % to 10 % of full load current (if required). • Material of Resistance Element : We offer resistor elements made from any stainless steel or FeCrAl or Cu-Ni or Ni-Cr or any other resistance material as per requirement. Normally formed stainless steel element are best and economical for NGRS. • Temperature Rise Allowed: Normally Temperature...