This counts the seconds in which this E1 port had at least one errored block. As the seconds-based counterpart to EB, ES on this port tells the admin how much of the elapsed time was affected by any error at all, the number typically compared against a G.826-style error-second-ratio threshold for the circuit -- and this specific object covers a closed 15-minute history bin -- a fixed, final count for one completed interval rather than something still changing. For example, over that completed 15-minute bin, this E1 port might show this counter at 1 -- a concrete count the admin can trend against neighboring bins to judge whether the condition it reflects is worsening.