* Eskom declares second emergency in three months
* Key industrial users asked to reduce usage
By Olivia Kumwenda-Mtambo
JOHANNESBURG, Feb 20 South African power utility Eskom declared a second emergency in three months on Thursday, saying the national system was severely constrained after outages at some of its generating units.
The state-owned utility is battling to keep the lights on in Africa's largest economy as it races to build new plants and meet rising demand.
Eskom said the emergency would last until 1900 GMT on Thursday. It last declared a power emergency in November, which was the first in five years.
The utility said it was making every effort to avoid "load shedding", as rolling blackouts are known in South Africa, and had asked its key industrial customers to reduce their usage by at least 10 percent from 1200 GMT.
"We are hopeful that by applying these measures, coupled with support from all electricity consumers, we will achieve the required load reduction necessary to protect the national grid," the utility said in a statement.
Mining group BHP Billiton said it would reduce power to its aluminium smelters in line with its agreement with Eskom to stabilise the national grid at times of stress. It receives no compensation for the disruption.
"We received the request, we will be turning down" spokeswoman Lulu Khumalo said.
The power emergency has arisen at a time when most mines in the platinum industry are not operating due to a strike. Workers at Anglo American Platinum, Impala Platinum and Lonmin downed tools four week ago, demanding higher wages.
South Africa last suffered a power crunch in 2008 that forced the closure of mines, factories and smelters for days, costing the economy billions of dollars in lost output.
More blackouts could also hurt the ruling African National Congress ahead of general elections in May.
Eskom provides 95 percent of South Africa's electricity.
Medupi, one of Eskom's new coal-fired plants, is expected to deliver its first power in the second half of this year, after several delays, and the current situation underlines the importance of meeting this deadline.