Buy capping from your electrical wholesaler, incase u don't know, capping is a convex tinplate cover maybe a yard or so long by varying breadths usually 1/12" or so, narrower or broader is available if required simply nail or screw-fix to cover the exposed cables and plaster over as normal. To do it correctly it should be bonded to your electrical earthing system ensuring that if it comes into contact with any exposed cables it will lift the fuse/breaker.
Granted u dont earth plastic but i think you will find that many Local Authority's/Council spec's was for metal capping and therefor required bonding....hence i was just covering worst case scenario's as metal capping does exist
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