Ranco etc burned out?

Pics from the tear down:

http://www.spyderrobotics.com/images...ernal_case.jpg
http://www.spyderrobotics.com/images.../case_rear.jpg
http://www.spyderrobotics.com/images...ndbox_back.jpg
http://www.spyderrobotics.com/images...rd_cleaned.jpg
http://www.spyderrobotics.com/images...lay_melted.jpg
http://www.spyderrobotics.com/images/ranco/relay.jpg

Here are my thoughts:

I do not agree with the wiring of this unit. This is the recommended wiring in the manual and is also displayed online in several tutorials. However, in their method the hot comes in and is connected to the upper contacts. These contacts go to the power transformer which runs the microcontroller. Then they have you use a jumper wire to run from that contact to the relay Common contact. The outlet is then wired to the Normally Open contact. The reason I don't agree with this is because the entire power now has to go through the jumper wire which is not as secure of a connection and could add problems. I feel the better solution is to run the hot from the incoming plug directly to the Common on the lower relay. The jumper would still be in the same location. In this method power goes directly to the higher power contacts which gives better transfer and the jumper only needs to provide the small current needed to supply the transformer.

Alternate wiring. If you kept with the same factory recommended setup I would replace the jumper wire with a pigtail fuse which would give additional protection against failure.

This is a single sided pcb. There are no traces on the top side of the board so all current is on the lower side. Looks like they have flowed solder on top of the copper to increase the current capability. This board took alot of current. This was not a short circuit within the thermostat as the neutrals are out of the way. This looks like dead short scenario or simply the pcb trace and solder combo was not sufficient to carry the current.

I removed the relay and opened it up to see if the contacts showed signs of excessive wear. The relay is actually well constructed and in this case the contacts showed no wear. No signs of excessive arcing. I do not feel the relay was the cause of the failure.

Tough to say if the heater itself had a short causing this or if truly the circuit boards are just not rated to handle that amount of current and this is the result. The pcb was so hot that the blackened area basically was dust and was a gaping hole in the board. Since these units do not have fuse protection I would recommend anyone using a oil filled heater to use a power strip or surge protector that has a circuit breaker connected to the wall as they can trip faster than the home breaker in most cases.

I did send an email off to Ranco which I believe is now owned by Schenider Electric about the wiring diagram to see if they have a reason for their recommendation. The wiring was a not a contributing factor in this failure.

Disclaimer: This post is my personal opinion and is meant for informational purposes. :)

-Dion Brewington

Also apparently there are a few models of Ranco out there. In a previous post I submitted a link showing the ETC21 model which had a rating of 9.8 amp resistive which is below what a oil filled heater uses on high. However, this model was actually a ETC1110 which is rated at 15 amps resistive so it was rated for this application. Here is a new link to the manual for this model.

http://www.etcsupply.com/manuals/ran...structions.pdf