Eskom's troubles have spread, affecting even its largest coal supplier, and raising the question as to whether being invested heavily in business with the electricity utility was the best thing for domestic coal suppliers. Exxaro, Eskom's largest coal supplier, was downgraded this week by ratings agency S&P Global Ratings after Eskom's and South Africa's sovereign credit were downgraded to junk status last week. S&P said that despite Exxaro's strong balance sheet and positive outlook Exxaro was vulnerable to Eskom because of its "concentrated customer profile".

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