Control Panel (Safe start interlock)
If you have not already done so, make sure to visit part 1 and part 2 of the control panel assembly instructions. This article assumes that you've assembled and wired your control panel as outlined in the previous two parts.
An interlock is a device used to prevent a machine from harming its operator or damaging itself. Safe start interlocks are typically found on machinery that may be dangerous. It ensures that the machinery cannot be turned on unless certain conditions are first met. For example, most vehicles can only be started with the gear shift in park.
Adding a safe start interlock to our control panel will avoid harming the pumps, heating elements, and the operator. Whenever using the control panel as built in part 1 and part 2, before it is turned on you must check to make sure that the ELEMENT SELECT and PUMP switches are all in the OFF position. The pumps we use can be damaged if run dry and the heating elements should only be used when submerged in liquid. (While the ULWD elements we use will not actually break if fired up "dry" like regular elements, it's still best that they be used correctly).
In addition, from an ergonomic/process control standpoint, when power is cut to the panel (either on purpose or because of a power outage) the pumps and heating elements should not automatically come back on when power is reintroduced (sometimes suddenly).
Our safe start interlock will therefore only allow the control panel to power up if the pump and element switches are all in the OFF position. If any of the three switches is in the ON position, the control panel will refuse to power up: Turning the power key switch will do nothing. This interlock will also ensure that if power is ever cut, the pumps and elements will stay off until the operator turns them off and then resets the power.
This may seem like an unnecessary precaution to some people so you may consider this article as optional. The extra parts required however are inexpensive (approximately $10 in total) as all we need to add is one 8-pin relay with 120VAC coil and an 8-pin socket. We hope that most people building a control panel like ours will spend the extra effort to put in a safe start interlock. It's quick and easy to do. If you build your control panel exactly like ours, you already have most of the other parts you need.