Finding and using isolation points before you start, handling a leak or burst safely, and thawing a frozen pipe without damage.
Knowing where a job's isolation points are, and using them promptly and gently, prevents most minor plumbing faults from becoming floods.
Before you start
Before starting any repair, find out what the problem is and how you would isolate the relevant supply — water, gas or electricity — if something went wrong while you work. On a larger job, also confirm the scope and price, the access you need, and whether there are any vulnerable occupants in the property, so nothing is a surprise once work is under way. Knowing where the main stop tap is means the whole water supply can be shut off quickly in a genuine emergency, and it is worth showing a customer where their own stop tap is and how to close it, since that simple knowledge limits damage if a pipe fails when you are not there.
When you need to isolate now
If a pipe unexpectedly releases water and the relevant isolation valve can be reached safely with no electrical hazard present, stop work and close it before doing anything else — do not keep working with a bucket underneath, and do not open outlets to try to help it drain while the supply is still live. If there is no local service valve on the fitting you are working on, isolate further back instead, such as at the main stop tap. Where a burst pipe is spraying near sockets or a wet consumer unit, keep people clear of the area, stop the water only if the valve can be reached safely, and arrange competent electrical isolation rather than touching a wet consumer unit or its main switch yourself.
A stop tap that will not cooperate
An old brass stop tap can seize, and forcing it with a long bar or heat risks shearing the valve and causing the flood you are trying to prevent. Ease it gently to and fro instead; if it still will not operate, isolate the supply at the external stop tap or through the water supplier, and advise the customer that the faulty internal stop tap should be replaced.
A frozen pipe
Where a hard frost has left no water at any tap, thaw the frozen section gently — warm cloths, a hot-water bottle or gentle warm air — and never with a blowtorch or other naked flame, which can scorch or split the pipe and start a fire. Watch for a split as it thaws, since the ice may already have cracked the pipe before you started.