IM-P402-72 AB Issue 6
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5.1 Screen connection
An earth current loop is created if a wire or screen is connected between two earth points,
which are at different potential (voltage). If the instructions are followed correctly, then the
preamplifier and controller screen will only be connected to earth at one end.
Note: The PA20 earth terminal is a functional earth rather than a protective earth.
A protective earth provides protection from electric shock under a single fault condition. This
product has a double insulation and therefore does not require a protective earth.
A functional earth is used in order for the product to operate. In this application, the earth
(tank /boiler shell) is used as the common of the probe / preamplifier. It also provides a sink /drain
for any electrical interference.
Ensure that the screen is connected to the earth terminal of the PA20 and to the common
terminal of the controller.
Ensure the common terminal of the controller is not internally earthed. (All Spirax Sarco boiler
controls are internally isolated from earth).
The common terminal of the controller must only be earthed via the PA20.
CAUTION:
Do not connect the common terminal to an earth local to the controller. To do so may
induce an earth current loop, which may reduce the performance or damage the
product.
5. Wiring diagrams
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