why in case of dc neither of terminal is at zero potentialpotential while in case of ac ground is at zero potential ,let us consider negative terminal of the dc is at zero potential then why we use two fuse in case of Dc ,while in case of Ac we use only one.
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2Hi, welcome to EE Stack Exchange! I'm not sure I understand your question I'm afraid. Could you please try to rephrase your question? – jramsay42 Aug 24 '18 at 12:12
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Some of us don't use fuses at all. Some use one. Two is probably some redundancy mechanism. – Aug 24 '18 at 12:17
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in case of dc why neither of terminal is at zero potential... – Inhamullah Khan Jadoon Aug 24 '18 at 12:23
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1One terminal may be at zero potential. Ground is just the reference zero voltage in a circuit. Is there a particular reason or situation you are referring to? Like maybe physical earthing is what you are talking about? – jramsay42 Aug 24 '18 at 12:31
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one thing is confusing me whether ininhe case of dc the negative terminal is at zero potential or not.. – Inhamullah Khan Jadoon Aug 24 '18 at 12:40
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It can be. Voltage is electrical potential difference, so it only makes sense to talk about the difference in potential between two points. We tend to choose a single reference point in a system and take all measurements relative to that, we call this ground. – jramsay42 Aug 24 '18 at 12:43
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Think of how you measure things with a voltmeter, we need two probes to measure across a component. It is convenient to leave one of these probes in a fixed position, then take all other measurements by touching the other probe to various nodes in our circuit. This fixed position is ground and could be anywhere, including the negative terminal of a DC voltage source. – jramsay42 Aug 24 '18 at 12:45
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Think of an Insulated Earth Return vehicle - the chassis is not connected to the battery negative for safety reasons, so both the live and the return are fused and fault-finding can be REALLY challenging if a fuse is blowing due to a wire touching the chassis - there are TWO faults to find... – Solar Mike Aug 24 '18 at 12:52
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1The statement "why in case of dc neither of terminal is at zero potential" does not make sense as it is written. Potential is a relative measurement. If the DC voltage is zero, the terminals are at zero potential with each other, after all. You need to specify what reference point you are using for the zero potential conclusion. Concerning "while in case of ac ground is at zero potential" - again, what is your secondary point of reference? It is illogical to define a single point as "zero potential" without a reference. – Adam Lawrence Aug 24 '18 at 13:02
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Also, concerning your fusing statements - there *are* AC applications where multiple fuses are required by regulatory standards. Not all AC applications use a 'hot' and a 'neutral' after all - some are phase-to-phase with an earth. – Adam Lawrence Aug 24 '18 at 13:10
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I wana say let say one terminal of dc source having potential 10v , with refernce to this terminal other may have some potential or it will have zero potential – Inhamullah Khan Jadoon Aug 24 '18 at 13:17
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1@Inha: See my answer to [Actual electric potential at terminals of battery](https://electronics.stackexchange.com/questions/355695/actual-electric-potential-at-terminals-of-battery/355706#355706) and see if that answers your question. – Transistor Aug 24 '18 at 14:03
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Possible duplicate of [Difference between negative terminal and copper ground?](https://electronics.stackexchange.com/questions/51478/difference-between-negative-terminal-and-copper-ground) – Blair Fonville Aug 24 '18 at 15:01
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There is no such thing as "zero" potential, it is all relative.
Usually what people use as a reference is an earth ground connection, which is a metal spike rammed into the ground. The earth is sufficiently low-resistance that the voltage between the metal poles in adjacent buildings is low enough to be of no practical concern, e.g. when you run a shielded cable like an antenna to your neighbor and connect the shield to ground on one side, you don't get a high voltage between the shield and the ground on the other. For longer runs of cables (e.g. a TV cable running for a few kilometers), you can measure a potential difference there.
Earth ground is also used for protection in AC wiring, but by design it is never used as a conductor in normal operation. Instead, a typical setup connects neutral to ground in one place in the house, and then a GFCI is placed between that connection and any outlets that will disconnect the power as soon as a non-zero current is measured on the earth ground connector.
The local substation contains a transformer, which isolates the local neutral potential from the electrical grid, which may use a completely different "zero" point. The electrical grid only distributes three different "hot" phases, there is no point in having a neutral wire there.
Similarly, DC power supplies also isolate the DC voltages from the potential of the outlet, and leave them floating. If you connect the negative DC supply to earth ground, the positive supply will be at a higher voltage than the ground, and if you connect the positive supply to ground, the negative will be at lower voltage than ground.
You need to take this into account when designing electronics that are to be used with long cables. For example, if you look at "Power over Ethernet" designs, there are "isolating" and "non-isolating" variants, the latter only to be used if the device will have a plastic enclosure and no external connectors.
For anything else, the building infrastructure takes care of most of the problems.
Simon Richter
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