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Instrument earth

Created by: Ashok Naphade at: 5/26/2014 9:14 AM (2 Replies)

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5/26/2014 9:14 AM	Rate (0)
Ashok Naphade Bronze Member Joined: 6/8/2013 Last visit: 12/4/2025 Posts: 160 Rating: (9)	Hi, What is the basic diff. in Instrument earth and power earth.

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5/26/2014 9:31 AM	Rate (1)
jeet2907 Posts: 235 Rating: (52)	Ashok Naphade Hi, What is the basic diff. in Instrument earth and power earth. Dera Ashok Naphade, Ther are major differences in function between the safety or protective earth, the instrument earth, and an IS earth. The main purpose of a safety earth system is to provide a good solid return path from the point of a fault back to the point of origin of the power - usually the power supply neutral point - so that protective devices see a nice fat fault current and trip immediately. Provision of a separate protective earth conductor in a cable makes sure that the fault current flow is as direct as possible and doesn't take detours through other paths - such as fortuitous routes via metalwork or other services. Instrument earth and IS earth need to be considered together but kept separate - and both need to be kept separate from the safety earth. The IS earth and instrument earth systems will be connected together at the barrier rail, since a load such as a solenoid valve will be connected between controller output and earth, and the "earth" may be made at the barrier. However, this connection needs to be done in such a way as to give only a single point of earthing for any one instrument circuit. The instrument/IS earth system will in turn be connected back to the power supply main earth or neutral point by a single circuit (maybe using 2 parallel conductors for ease of testing, but effectively only one current path). Codes will usually specify the maximum resistance of this connection, typically no more than 1 ohm. Multiple connections between the IS/instrument earthing system and the safety earth system may allow fault or stray earth currents to flow in the IS-neutral connection, and raise the potential of the IS system to an unsafe level. They will also generally cause merry hell with any instrument results.
jeet2907 Posts: 235 Rating: (52)
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5/26/2014 12:27 PM	Rate (0)
Chetan2490 Posts: 149 Rating: (16)	Ashok Naphade Hi, What is the basic diff. in Instrument earth and power earth. Dear , i hope thislinkwill help you.
Chetan2490 Posts: 149 Rating: (16)
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Instrument earth

Created by: Ashok Naphade at: 5/26/2014 9:14 AM (2 Replies)

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