I remember it all too well, sitting in our brand new motorhome and looking over the interior, grateful to be so fortunate to have such a nice RV. Then I noticed it, a hum, was it my tinnitus acting up? No, this was different, it was a lower pitch and even more annoying. After some hunting I located the offending component, it was the RV Automatic Transfer Switch! Since it was in the bottom of the pantry and we did not have carpet to dampen the sound, it was extra loud. I tried the usual potential remedies, tightening the screws on the terminal block, blow air in and around the contactors to remove any dust or debris, but nothing worked. I even tried to insulate the area but to no avail. By the end of camping season I had had it and needed to replace this RV automatic transfer switch, but with which one? I definitely did not want to repeat the problem.
So after some investigating and a few phone calls to technical departments, I was filled in with the issue. An RV automatic transfer switch is basically a 3 way switch, redirecting power from a generator to shore power and vice versa into the RV’s power center, never allowing either power source to come in at the same time. To do this the transfer switch uses relays, and in the case of my unit, AC powered relays. So to keep things somewhat simple, in an AC relay, the alternating current supply looks like waves on the ocean when seen through a scope. As that wave goes above and below flat calm, the current flowing through the coil in that relay becomes zero. The resulting loss of magnetism in the coil for just a few milliseconds creates the hum we hear in the transfer switch. There are a few more factors that can contribute to the hum, but suffice it to say if it is bothering you, you will need to replace the unit.
What’s the solution you ask? Thankfully, RV automatic transfer switch manufacturers are now selling a version that uses DC relays which have virtually no hum. With this new technology, versions like the Progressive Dynamics PD52DCSV Automatic Power Transfer Relay Switch even feature surge protection as well as open neutral and reverse polarity protection. While transfer switches with DC relays are slightly more expensive due to the added circuitry required, the benefit far outweighs the hum AC relay versions can cause. Since I already had a surge protector, I opted to go with the ESCO Lyght series of RV transfer switches and could not be happier. The hum is gone and I am now able to enjoy peace and quiet.