Introduction to DC Voltage Ratings
When designing any system that uses batteries, it’s extremely important to use DC rated components, especially when working with solar arrays. Using incorrect component ratings can be a safety and fire hazard. Not only should circuit breakers and fuses have correct voltage ratings, but relays, switches and other components and devices should as well. Failure to use the correct ratings can be dangerous and can cause extensive system damage.
AC vs. DC Voltage Ratings
Although many components have dual AC and DC voltage ratings, not all components do, and most have much lower DC ratings than AC. This can be seen through reduced voltage, current, or both. As an example, Square D QOU circuit breakers are rated 120VAC and 48VDC. These breakers would be fine for a 12V or 24V nominal DC system, but not 48V as the battery charge voltage can be as high as 60V and the array voltage higher than 80V. Similarly, the CBI QY breakers come in an 80VDC rating and a 125VDC rating, and can be specially rated to 150VDC for solar arrays. Using an 80VDC breaker on a 48VDC nominal system might not be adequate as the solar array Open Circuit Voltage (Voc) can be in excess of 88VDC before temperature correction. Refer to SunWize’s tech note on Temperature Effects on PV Modules. Fuses can also have misaligned ratings. Popular glass AGC fuses actually have no DC rating at all. On the other hand, ceramic ABC fuses (which have the same physical dimensions as the AGC fuses) have a 250VAC rating and a 125VDC rating. Mechanical relays can be even more complicated, as they might have DC coils but no DC rating for the contacts. While a relay may be rated 6A at both 250VAC and 250VDC, another is only rated 10A at 110VAC and 30VDC. The ratings are become further complex when determining whether the load is resistive or inductive. That same 10A is now only rated 7A for inductive loads.
In addition to voltage ratings, overcurrent protection devices also have maximum interrupt current ratings, and these may vary with the voltage.Using the Square D QOU breakers again as an example, they are rated 10,000AIC at 120VAC, but only 5,000AIC at 48VDC. The Amp Interrupt Current (AIC) rating is important, as batteries can produce a tremendous amount of current when short circuited. The battery short circuit current rating can typically be found on the battery spec sheet or by contacting the manufacturer. Generally, small batteries have lower currents than larger batteries.As an example, the Deka 8GU1 battery can produce 1175A when short circuited, the 8G27 can produce 2348A, and the 8G8D battery 5794A. This can get even more complicated when the batteries are wired in parallel, as the currents can get added together; however, the actual shirt circuit current will be less as the resistance of the parallel wires will reduce the maximum combined current.
When working with solar arrays it is critical the voltage remains consistent. Not all components have dual AC and DV voltage ratings. It is because of this that extreme care must be taken; failure to use the correct component ratings will result in hazardous situations.