If you’ve taken our Level I Thermographic Applications course or our Electrical Specialty course you’ve been taught that the NFPA-70B Recommended Practice for Electrical System Maintenance specifies electrical apparatus operate at 40% of rated current as a minimum for electrical IR surveys. What you might not have heard is why that’s the case. We discuss it at a relatively high-level view, but the opportunity isn’t there to drill deeper. However, in this tech tip, we have the chance.
Combining Ohm’s Law and Watt’s Law we arrive at a formula that should be familiar to electrical thermographers describing what is referred to as “The Joule Effect”.
This can also be referred to as “ohmic heating” or “resistive heating” and is how many electrical heating systems work, for example, electric stoves, toasters and space heaters. Current that is passed through a resistance generates heat. The amount of heat generated rises at the square of the current. This relationship is precisely why there’s a minimum load requirement for electrical surveys.
Below is a table showing the relationship between the percentage of rated current and generated fault power. This example is for a 100-amp circuit, with 100 milliohms of fault resistance.
Something worthy of note here: Most actual fault resistances found in the field are a few microohms of resistance. Milliohms were used to simplify the math for the sake of this discussion. For example; in a 100-amp circuit, 40% rated current is 40 amps. As you can see from the table, at 40 amps the level of fault power is only 16% of the potential full power. As happens in exponential relationships, as the current increases above 40%, the change in the percentage of fault power rises more and more. Truth be told, for many electrical systems 40% of rated current might not be sufficient for very low-grade anomalies to be detected. See the chart below.
Resistance, current and heat generation are in an exponential relationship, but this isn’t to be confused with actual component temperature. The relationship between current and temperature is less than exponential but greater than linear. Electrically, as heat increases resistance also increases. So, as we increase the load on electrical components the temperature is impacted by both of those phenomena. As shown above, once we have a system load above the minimum suggested level of 40%, the slope of the rise steepens. Optimally, above 80% rated load is where inspections should be performed, as the likelihood of detecting lower grade resistive failure modes increases.
When considering operating time, opinions vary on how long a system should be running under load before an IR survey. A good rule of thumb is approximately half an hour running loaded greater than 40%. Obviously, depending on many variables, success can be had at shorter operating times. The things that will impact the required time are the ambient temperature in the environment where the equipment is installed, the mass of the electrical system components, and the actual current level.
As you continue in your journey of electrical IR inspections, remember the 40% Rule, you now know why it applies. Be safe and Think Thermally®.