Electricity has been turned off around Bernards Heath and Batchwood.
Residents in parts of St Albans woke up this morning without any electricity in their homes.
A problem was reported to UK Power Networks at about 4.30am today (June 20), which was found to be caused by a faulty piece of underground equipment.
At 9am the supplier turned off the electricity in 170 postcodes around AL1 4 and AL3 5 - Bernards Heath and Batchwood - to fix the problem.
A spokeman from UK Power Networks said: "Our engineers are working quickly and safely to complete the repair so that supplies can be restored as soon as possible.
"We appreciate how difficult it is to be without power and apologise for any inconvenience caused."
The electricity supply is expected to return to normal by about 1.30pm.
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