Every heater Dalton makes runs through our 8-point gauntlet of testing and inspection. First up in our series of electrical conformance tests is “Insulation Resistance” and goes by many names Megohm, Megaohm or Megger.
Dalton uses this simple test to begin with as our downstream tests likely fail if the heater does not pass the megohm test. The Megohm testing device applies a high direct current (DC) voltage, 500 VDC in our case, between a conductor and one or several other conductors for a few seconds. A high resistance value is expected – typically greater than 500 Megohms with infinity being the ideal meter reading. Based on our many years making and testing heaters, we know that values less than 500 megohms indicate the MgO is “wet”. Wet heaters are oven dried for at least 4 hours at (115° to 125°C) and return to the first step in our testing sequence.
Dalton also knows that “wet” heaters create headaches for end-users too – tripped breakers, relays and wasted time trying to figure out the problem. They take great care to pack our fully dried heaters in sealed bags with desiccant and surround them with a protective wall of form fitting foam. This ensures their heaters arrive at your or your customers door the same way they left regardless of the rough handling along the miles of conveyors and hand-offs made in transit.
Some end-users megohm test their heaters prior to installation to ensure insulation is dry and effective. This simple and fast test identifies heaters that have picked up too much moisture in storage or transit and need further drying prior to installation.
It’s also one of the first things to consider when a customer is having problems after extended shutdowns (like July 4 or Christmas). The best way is start up at half voltage to allow the heaters dry themselves for 15-30 minutes. Newer controllers have soft start “Drying Sequences” that apply lower voltage to the heaters for 15 minutes or so to allow them to safely heat up and dry out prior to applying full voltage.