Automatic Transfer Switches (ATS) for RV’s come in various sizes and configurations. Let’s see what we’re switching and then what we will need to do it.
RV’s come in two basic electrical service configurations, 30 amp and 50 amp. The 30 amp is just an overgrown extension cord that carries 30 amps of 120 volts from a heavy duty 3 wire receptacle and plug to the camper electrical system. It has one hot wire, one neutral wire and one ground wire. A 50 amp system is a little different. It has a heavy duty 4 wire receptacle and plug and features two ‘legs’ of 120 volts (totaling 240 volts) with each leg capable of 50 amps. The neutral and ground wires are also in this setup. It is an undersized version of the 200 or 400 amp system that feeds your house or small business.
Now, what is an ATS and what does it do? The ATS switches between two sources of electrical power without letting either source connect to the other. With no ac power in a coach, the ATS is in the ‘shore cord’ position. If you plug the ‘shore’ cord in, the camper will have power. However, if you start the generator, it will initiate a time delay to warm up the engine and then disconnect the shore cord and connect the generator. If the generator stops, the ATS will immediately shift to the shore cord position again. At no time are the shore power and generator power connected together. (It makes lots of arcs and sparks if they do connect). ATS’s are purposely designed to physically and electrically prevent the two sources from coming together.
Single relay ATS. This ATS consists of one, 30 amp, double pole, single throw, 120 volt, DC relay and a diode/time delay circuit. To explain that – double pole means that the relay has two sets of 30 amp contacts, so it can connect both the black and the white wires to their respective sources of power and the green ground wire is not switched. Single throw means that the relay is connected to one source (shore cord) unless the electro-magnet coil is energized. When the coil is energized, it opens the shore cord contacts and closes the generator contacts. 120 volt means that the coil of the relay operates on 120 volt and, in this case, it operates on DC voltage. RV ATS’s use dc voltage because it connects and disconnects sharply. AC voltage will make the relay ‘chatter’ as the voltage drops off during the shift from generator to shore cord. The diode/time delay circuit may be a heater and thermo switch as is found in some home style heat pumps or a small printed circuit board. In either case, the circuit delays the switching to generator to allow the engine to warm up a little.
Two relay ATS. This model works exactly like the single relay one above but has two relays in it. The relays are wired so that one relay has to completely switch and operate a ‘micro-switch’ to allow the other relay to operate. This seeming inconsistency is explained by having the loads that the ATS is furnishing separate from each other. This ATS is usually used in coaches that have generators with two 120 volt lines coming out of them (not 240 volt circuits). One circuit feeds the rear air conditioner and the other feeds everything else in the coach. Two relay ATS’s are also used to switch from ac to inverter or solar power
Contactor type ATS. This is usually a 70 amp, heavy duty, dual contactor, three or four wire switching device with a solid state or printed circuit time delay board. They differ from light industrial level ATS’s mainly by their lack of arc chutes to reduce excessive arcing during the transfer cycle. These models normally switch both ‘hot’ legs of the 240 volt circuit, the neutral and (maybe) the ground as well. This unit usually has a DC coil system for chatter free switching. Some of these ATS’s are only energized during the switching cycle. When the cycle is complete, a latching system holds the contactor in the new position.
Unless you are electrically oriented, I would highly recommend that you seek professional help to install, troubleshoot or maintain any ATS. If you want to work on these little monsters, insure your spouse/significant other has your life insurance policy paid up to date.
Generator Jim