The PP-20 has patented Relay Circuit Protection (tested to over 400,000 cycles at rated load), whereas the MP20 does not. The relay in the PP20 is designed to switch AC currents only and will not switch a DC signal inputs to EMS, HVAC, LC&D, etc. If a DC signal is required then us the MP20 or a low voltage sensor with the "R" option (e.g. CM PDT 9 R).
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