* Third power emergency in two weeks
* Eskom says heavy rains affect coal quality
* Rolling blackouts a "strong possibility"
By Olivia Kumwenda-Mtambo
JOHANNESBURG, March 6 (Reuters) - South African state power utility Eskom declared a new power emergency on Thursday and said there was a "strong possibility" it would have to implement rolling blackouts for the first time in six years.
Eskom's grid has come under strain because of heavy rains over power stations in eastern Mpumalanga province that have made coal supplies wet, spokesman Andrew Etzinger said.
Most of the utility's power stations are coal-fired.
"The power system is very tight," Eskom said in a statement.
It is the third time the utility has declared an emergency in the past two weeks.
South Africa's big industrial customers have been asked to cut their usage by 10 percent while other customers been told to turn off geysers and swimming pool pumps to prevent rolling blackouts, known locally as 'load-shedding'.
The mining and smelting industries are very power intensive and BHP Billiton is Eskom's single biggest customer. Etzinger said the emergency would likely be lifted at 9 p.m. local time (1900 GMT).
Rolling blackouts caused misery for millions of South Africans during a severe power crunch in 2008, which cost the economy billions of dollars in lost output.
Eskom said that, should rolling blackouts become necessary, it would use schedules published on its website. ()
Eskom has warned that power will remain tight until new generation capacity comes online. Its new Medupi plant is expected to come onstream in the second half of this year after several delays.