The device, which was rigged to a timer, was attached to a steel pylon located between a key thermal power generating facility and the main control centre in Colombo.
The high-tension line supported by the pylon forms the backbone of the country's power transmitting system.
The blast was heard five kilometres (three miles) away but caused only minor damage to the pylon, and no disruption to power supplies, officials said.
No one was hurt in the explosion.
"The line is energised and there is no immediate threat of the cables collapsing," a spokesman for the Ceylon Electricity Board said.
"We have not switched off the supply, but we will start repair work soon.
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The defence ministry blamed the Liberation Tigers of Tamil Eelam (LTTE), who have been fighting for an independent Tamil state in the north and east of the island since 1972.
Engineers said the bomb could have destabilised Sri Lanka's entire electricity network by disconnecting the thermal generators from the national grid.