ABB REL 551 2.3 Series Installation And Comissioning Manual page 22

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Overview
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The mechanical and electrical environmental conditions at the installation site must be
within permissible range according to the technical data of the terminal. Dusty, damp
places, places liable to rapid temperature variations, powerful vibrations and shocks,
surge voltages of high amplitude and fast rise time, strong induced magnetic fields or
similar extreme conditions should be avoided.
Sufficient space must be available in front of and at rear of the terminal to allow access
for maintenance and future modifications. Flush mounted terminals should be mounted
so that terminal modules can be added and replaced without excessive demounting.
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