Absolutely. If you transmit at 500 watts while the SDA-100 is tuning you can damage your DB-18 EHU. The interruptor doesn't provide the actual protection. It just forces you to use a separate key out cable between the K3 and KPA500. This is the cable that you would then route through a SteppIR control device which could be either the $82 Tuning Relay option for the SDA-100 or the $78 N8LP SteppIR Tuning Relay external device. You can get very specific advice at the SteppIR Yahoo group.