LONDON--(BUSINESS WIRE)--AM Best has affirmed the Financial Strength Rating of A- (Excellent) and the Long-Term Issuer Credit Rating of “a-” (Excellent) of National Grid Insurance Company (Isle of Man) Limited (NGICL or the captive), a captive insurer of National Grid plc (NG). The outlook of these Credit Ratings (ratings) is stable. The ratings reflect NGICL’s balance sheet strength, which AM Best assesses as very strong, as well as its adequate operating performance, neutral business profile