Noise and Grounding 401as it deprives you of your best weapon against mains noise coming in and circuit RF
emissions getting out. In Class II the external chassis may be metallic and connected to
signal 0 V as often as you like.
If a Class II system is not connected to ground at any point, then the capacitance between
primaries and secondaries in the various mains transformers can cause its potential to
rise well above ground. If it is touched by a grounded human, then current will fl ow, and
this can sometimes be perceptible, although not directly, as a painful shock such as static
electricity. The usual complaint is that the front panel of equipment is “ vibrating ” or that
it feels “ furry. ” The maximum permitted touch current (fl owing to ground through the
human body) permitted by current regulations is 700 μ A, but currents well below this
are perceptible. It is recommended, although not required, that this limit be halved in the
tropics where fi ngers are more likely to be damp. The current is measured through a 50 k
resistance to ground.
When planning new equipment, remember that the larger the mains transformer, the
greater the capacitance between primary and secondary and the more likely this is to be a
problem. To put the magnitudes into perspective, I measured a 500 VA toroid (intended for
Class II usage and with no interwinding screen) and found 847 pF between the windings.
At 50 Hz and 230 V, this implies a maximum current of 63 μ A fl owing into the signal
circuitry, with the actual fi gure depending on precisely how the windings are arranged.
A much larger 1500 VA toroidal transformer had 1.3 nF between the windings, but this
was meant for Class I use and had a screen, which was left fl oating to get the fi gure above.
13.4.1 Warning
Please note that the legal requirements for electrical safety are always liable to change. This
book does not attempt to give a complete guide to what is required for compliance. The
information given here is correct at the time of writing, but it is the designer’s responsibility
to check for changes to compliance requirements. The information is given here in good
faith but the author accepts no responsibility for loss or damage under any circumstances.
13.5 Mechanical Layout and Design Considerations....................................................
The mechanical design adopted depends very much on the intended market and
production and tooling resources, but I offer a few purely technical points that need to be
taken into account.