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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Steady and Transient Voltages
The first thing we need to understand is that voltage is by definition a potential difference, and despite being stated as an absolute number, actually implies an electrical gradient between two points. In a typical circuit, we implicitly measure and state the voltage at any point with respect to a certain reference rail, which we have designated as the system ground. However, in safety, we are concerned with voltages measured with respect to earth ground. That reference determines whether a voltage is considered hazardous (capable of causing electrocution) or safe. We ask why on earth is earth ground so important in safety? Because that is the safest level—we stand on it every day.
So from a safety perspective, ...

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