The formula for power in watts is P = I . E where I is current and E is voltage or is it?
FACT1 : Voltage is the potential for current to flow.
Theory : Voltage is also called "potential difference" or " electromotive force". As with any measurement we need to have a standard point of reference for voltage that is 0V and another point of interest. The standard point of reference of 0V is earth. Voltage is measured between earth and any other point. However this is not the voltage we should consider in power or work done calculations as voltage in itself does not do work or power in watts.
FACT2 : Current is the flow of electron.
Theory: Conventional current flows from a higher voltage to a lower voltage in a closed circuit. Current is a rate measurement that is number of electrons passing a point per second. It is plain to see that higher the potential difference more the electron flow and higher current. Current does work. Current is indirectly proportional to resistance. A definitive test in live testing for a closed circuit or a circuit doing work is to measure the current.
To summarize, Current only flows and can be measured in a closed circuit. Potential difference is only measured in an open circuit.
FACT3 : Voltage drop is the voltage that is dropped across a load in a closed circuit. Voltage drop is caused by a potential difference, current flow and a load resistance.
Theory : Voltage and Voltage drop are very different things which are sometimes mistakingly considered to be the same thing. Voltage can be regarded as a cause, and on the other hand, Current and Voltage drop is the effect only in a closed circuit.
Only loads have a voltage drop because only they are doing work. Switches donot have a voltage drop. It is voltage will you measure across a switch in a circuit.
The Load is the final peice of the puzzle. In a series circuit the voltage drop across a load is directly proportional to the Load resistance and current. There maybe one or more loads is series. In this case the higher the Load resistance the greater the voltage drop since current remains the same is a series circuit.
Voltage drop in a parallel circuit is the same as source line voltage in quantity but if a brach in a parallel circuit is opened by a switch the there is no current flow through that branch so no voltage drop but there will still be a potential difference or Voltage. The other branches will have voltage drop and the as an effect the total current drawn will be reduced.
For testing a series circuit we need to switch on and measure current and voltage drop across the load. The product of these two measurements will give you the power in watts. Voltage drop divided by current will give you load resistance.
Lets consider a series circuit with line voltage, one or more loads and an open switch what is the power consumed by the loads? OK that's easy liven the circuit and measure voltage drop across each load we should read 0V and use the E square by R formula we will get zero power consumed. Remember the switch is open means there is no voltage drop. So zero power is consumed. The switch is a control. However there will be a potential difference of line voltage across the loads when tested before and after the switch.
Now let's close the switch. Potential difference across the switch drops to 0V and there will be a voltage drop across each of the loads. Current will also be present. Potential difference should be measured using a loading meter to rule out any open neutral fault.
Protective devices are reactive temperature effected devices. PTC thermistor, bimetals, fuses are some examples of protective devices. They protect the load and the conductors from over current. With the appliance plugged in you should read 0VAC across these devices under normal conditions that's because they are normally closed. In cases of overheating the protective device will open and then you should read line voltage across its terminals.
In conclusion voltage and voltage drop are cause and effect respectively. Power in watts is the product of voltage drop and current, is the formula for appliance repairs. To definitely test a circuit we should do voltage drop and current tests.