Polarity Testing

hitchglitch

Replying to a topic on eath connections and polarity I went to check the Caravan Club book's advice on reverse polarity and found a statement that polarity testers should not be left connected in the caravan socket outlet.  We always leave our tester in
place because the neon lights act as an indicator that the site supply is still functioning.  There is no warning against doing this on the tester.  Does anybody know whether the CC advice is correct and, if so, why?  We have the tester that is illustrated
in the CC  Continental Sites Guide book.