Ponies Mason, Heston, Blue and Chuckles and a horse called Jerry were found dead at Ashton Hall Equestrian Centre on Sunday, 13 June.
Investigators from the local authority blame a faulty water feeder for the incident.
A Trafford Council spokesman said animal welfare officers and health and safety inspectors visited the riding school the next day.
“The first indications are that an electrical fault caused the accident,” she said, adding that the electricity supply to the barn had been turned off after the accident until repairs can be made.
RSPCA inspector Melissa Furey was called to the stables by a member of the public.
“There is nothing to suggest this is anything other than a tragic accident,” she said. “There were no marks on the horses and no signs of deliberate cruelty.”
H&H vet Karen Coumbe said: “Anecdotal evidence suggests horses are particularly sensitive to electric shocks because metal shoes conduct electricity, but also because they have four legs and a large heart.
“The electric current can run up a foreleg through the heart and down a hindleg. Thankfully, such cases are pretty rare.”
Tom Davin, who runs the riding stables, did not wish to discuss the incident.
This article was first published in Horse & Hound (24 June, ’10)