WorldWide Drilling Resource

39 WorldWide Drilling Resource ® OCTOBER 2014 Check It Under Load by Britt Storkson Owner, P2FlowLLC A friend of mine spent quite a bit of time and expense trying to find an elec- trical problem. With a voltmeter, he found he could measure voltage on the circuit with the circuit breaker turned on, but when he plugged in a light, nothing worked. Was this some spooky haunted house scenario? Were the electricity gods playing tricks on him? The answer is, none of the above. In his case, a loose wire nut had allowed the wires to “arc” within this nut and, over time, soot from this arcing built up in the wire nut. While soot is electrically conductive, it has a much higher resist- ance than a clean wire nut connection. What we have here is called a “voltage divider”. The formula for determining the voltage from a voltage divider circuit is R2/(R1+R2). R2 (resistor 2) divided by the sum (adding together) of both resis- tors. This formula yields a decimal value that is multiplied by the voltage you are feeding into the circuit. So, if both resis- tors are of equal value, the voltage you feed into this resistor string will be divided by 2. It’s the ratio of the two resistors that creates the voltage divider. One caveat: When dealing with elec- trical energy and resistance, one must be aware that heat can be generated and the components must be sized correctly to deal with the heat - sometimes a LOT of heat. However, to keep things simple, we won’t cover the heat (wattage) aspect of this equation. So, if the problem wire nut connec- tion presents a resistance of 10,000 ohms (this can be measured in the field) and the load presents a resistance of 144 ohms (a 100-watt incandescent light bulb at 120 volts needs to be 144 ohms resist- ance), then this resistor “string” would measure about 2 volts between the bad wire nut and light bulb. R2 = 144 ohms, R1 =10,000 ohms so 144/(10,000+144) would equal 0.014. Multiply this decimal by the 120 volts we are feeding into this thing, and you will get about 2 volts...not nearly enough voltage to run this light bulb. This can also be measured in the field. To do this, shut the light off by dis- connecting the neutral “return” path. With the voltmeter, measure the voltage from the “hot” lead to ground. It should meas- ure 120 volts (or whatever voltage you are feeding into it). Now reconnect the neutral wire to get a current flow. The voltage will drop considerably. Using Ohm’s law, if we know any two electrical components (the compo- nents being voltage, amperage, resist- ance, and wattage), we can calculate the other two. With high currents, even small amounts of resistance can create large voltage losses. Maybe you can’t measure less than one ohm of resistance, but you can calculate it using Ohm’s law. If you know the current “load” (amper- age) of the circuit and voltage, you can calculate the resistance. First, measure the voltage without the load. Then “load” the circuit and measure the voltage again. The difference (one voltage subtracted from the other) divided by the amperage of the load, will give you the load resist- ance. For high-amperage loads, it’s often in milli-ohms (thousandths of an ohm). In a high-amperage circuit, sometimes even one extra ohm of resistance can render the device inoperable. It can also generate considerable heat as the amper- age squared times the resistance = watts. So, when you run into a situation where you can measure voltage but can’t get the device to work, check the volt- age under load. Auto battery cables are famous for this problem. You measure a good 12.6 volts at the battery, and when attempting to start the car, the battery seems to “hold up” well. Now measure the voltage at the starter and attempt to start the car. If there is a con- siderable voltage drop, you know the cable is bad. Often, battery acid can seep into a cable and corrode it from the inside. The cables may look all right, but have a high resistance, which doesn’t allow the starter motor to work correctly. If an electrical device doesn’t work and the cause isn’t immediately obvious, be sure to check the voltage with and without load. You may be surprised at what you find. Britt Britt Storkson may be contacted via e-mail at admin@ worldwidedrillingresource.com & ! $ # & # " ! $ # & # & " & ! " & " $ # & & ! & " & & "! " ! & " " ! ! ! !% $ # "! !