Switches

The style and selection of available switches and outlets varies greatly from country to country. The important point in selecting switches for your control box is that they should be able to handle the load. Select commercial or industrial grade switches rated for 20 amperes, and double check that the voltage rating is 240V or higher (most are actually rated at 600v), whether you are making the 120V control box or the 240V version. These are not standard household switches, but should be available in an electrical supply store or a large hardware store. All wire used should be at least 12 gauge, and 10 gauge would be a better choice.

We use two types of switches in the control boxes we have designed, and it might be useful to describe them briefly for those unfamiliar with such things.

SPST. This is short for "Single Pole Single Throw". This is used when you need to close or open a connection in a single line to another single line. It is a simple ON/OFF switch and has just 2 terminals. SPDT. This is short for "Single Pole Double Throw", and is used when you need to close or open connections to either of two other lines. We've shown this latter type in Fig. 7-14, but SPST could be used instead. It has 3 terminals.

DPDT. This is short for "Double Pole Double Throw". In operation, it's exactly the same as if you has two SPDT switches joined together, and so has 6 terminals. This is used when such connections have to be made simultaneously, as leaving one closed when the other is open might lead to a dangerous configuration. These switches are used in both the 120 V and 240V control boxes.

The 120V control box has two leads to plug into two 120V wall sockets, and the 240V control box has one lead to plug into one 240V wall socket. Both have two power sockets to supply power to the elements. The plug and sockets can either be mounted directly on the box, or connected to it by a length of 10 gauge flexible cable. Each heater element has its own plug, appropriate to the voltage being used, and all you have to do is ensure that it's plugged into the correct control box socket. Color coding the plugs and sockets, or even using two different designs of sockets will help keep things straight. If they are plugged into the wrong sockets, it isn't dangerous, but the intermediate power settings will not be what you expect, because switch A will control element B, and vice versa!

Since no heat is generated inside the box, it can be sealed tight, and does not need any ventilation holes.

Mark the element plugs, sockets and switches clearly and, if using switches, attach a copy of the truth table, indicating the power provided by each switch setting.

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