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Appendix A – Safety principles
If the scanner must be powered from a mains supply, rather than a 12-volt battery, observe the
following safety suggestions.
Ensure the equipment is of good working order and electrical bonding is intact.
If an extension lead is to be used it must be properly constructed and in good condition; it must be a
three core cable (live, neutral and earth) and be used with 3-pin connectors.
For this reason we recommend that you always use your own leads which you know are safe. Do not
turn up at a site and rely on being able to find an extension lead available for your use.
This is important for two reasons:
• To act as a safety barrier between the operator and any live parts.
• If an internal fault should occur within the extension lead, mains voltage is conducted away
through the earth lead. The resulting current would trip a Residual Current Device (RCD), if
fitted, or blow a mains fuse.
An RCD is designed to trip at 30 mA of earth current which combined with a suitable low earth
impedance should result in a maximum voltage shock of no more than 50 volts at which the RCD will
then cut off isolating the equipment at zero potential.
If no RCD is fitted then the mains fuse would blow, however, this takes significantly longer (depending
on fuse rating) and can result in shock voltages of much higher values, unless an isolating step down
transformer with a centre tap is used in which case the maximum voltage present would be 55 volts.
A small voltage may exist between the instrument case and the ground when significant earth
current is flowing in the vicinity. This might be enough to be felt by the operator or the subject and the
following guide-lines should be followed.
• Do not set up for operation close to an electric fence, especially an electric fence control box.
• Do not set up for operation near the main earth point of an electricity supply installation.
• If another piece of equipment becomes faulty which is connected without an RCD, it would not
be protected by your RCD, it is possible then that dangerous voltages may appear on the earth
lead.
The only way to protect against any earth problem is to supply your own earth line from your crate and
to an earth rod or metal work close to where you are working.
Note: The safest manner of operation is from a 12 volts battery source.
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