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Double Throw Power Switching

Double throw relays like Kilovac's PD5C are used in a variety of "hot" or "power" switching applications, and they generally work fine. However, many application problems can be avoided by verifying that the relay was tested with the polarity and load conditions exactly like the application. In the aerospace industry, it is generally well known that switching un-synchronized AC loads is not recommended (see figure 28a).


Figure 28a.
Un-synchronized AC Loads - Generally not recommended

We found another application that took our experts some time to troubleshoot. This applica-tion required the charging and discharging of a capacitor. The double throw relay was used to switch a capacitor, which was in series with a resistor, between the plus and minus of the power source (see figure 28b.).


Figure 28b.
Improper placement of resistor for power switching

But when the relay was used to discharge the capacitor, an arc occurred as the plus side of the load was interrupted, and an arc with full power system potential followed the moving contact to the minus side of the power source, thereby causing a short. To remedy the problem, the resistor was removed from the moving contact, and resistors were installed on both the plus and minus sides of the power source (see figure 28c).


Figure 28c.
Proper placement of resistor for power switching

This reduced the current, and therefore eliminated the arc fault during contact transfer. Kilovac tests all of its hot switching relays with the positive polarity on the common contact, unless otherwise requested. The application above, and others that may involve switching between two power sources, require special testing to confirm a workable circuit design.