HIO16: Safety considerations
Note: This document is not yet complete. It may contain inaccuracies.
| By proceeding to use the HIO16 board you are indemnifying SPLat Controls, its agents and resellers against any claims that may arise due to death or injury arising from the use of the HIO16. If you are not prepared to provide such an indemnity please return the HIO16 to the supplier for a refund of the purchase price. |
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Voltage isolation
The primary safety consideration with any control element that is switching voltages greater than the Extra Low Voltage (ELV) limit is how well those voltages (for example 120VAC or 240VAC) are insulated from a human user. In terms of a board like the HIO16 this reduces to a consideration of:
a) The breakdown (isolation) voltage ratings of the input optocouplers and the output relays.
b) The physical spacing on the printed circuit board between "hot" conductors and "safe" conductors. This is commonly referred to as creepage distance, i.e. the distance across the normally insulating board that a current would have to "creep" in order to produce a shock hazard.
The optocouplers are rated at 5kV rms input to output isolation (1 minute). The full data sheet is supplied on the resources CD. Look for pc815_e.pdf.
The relays are rated for 4.5kV between coil ("safe") and contacts ("hot").
The following diagram details the creepage distance for various parts of the board.
(Diagram being prepared)
If you use metal mounting screws we suggest you use insulating tubular spacers (standoffs). It is advisable to use nuts and/or screw heads that do not extend past the metal pattern surrounding each mounting hole. Alternatively use insulating washers under the nuts or screw heads. We strongly urge you to use all 7 mounting holes to ensure firm mounting and to support the board while connections are made and unmade. The DK version of the board is supplied with snap-in nylon standoffs.
When mounting the board you must take into account the proximity of the exposed printed circuit conductors to the mounting surface (probably a metal surface).
Wiring (insulation) considerations
In many applications the input wring and the wiring to the relays will be at a dangerous voltage. At the same time it will necessarily have single layer insulation. Safety regulations in most jurisdictions, and plain engineering common sense, dictate that you should physically separate such wiring from the ELV wiring.
High currents and fusing
The output relays on the HIO16 are rated for high currents, up to 20A running and 80A inrush. You must make sure that the connecting wire you use is appropriately rated for the load current. You must also make sure the crimp connectors you use are selected to match the wire gauge, that they are rated for the current and that they are correctly crimped. Failure to observe the precautions could result in an overheated contact leading to a dangerous situation.
To prevent fault currents from overheating wires and melting the insulation it is mandatory in most jurisdictions that circuits carrying high currents be suitably fused to fault trip at a current less than or equal to the rating of the wires.