Demo, MySQL - rextester

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drop table tabtest;

CREATE TABLE tabtest
    (source varchar(8), voltage numeric,timestamp datetime);

INSERT INTO tabtest
    (source, voltage,timestamp)
VALUES
    ('CIRCUIT1', 2.4,'2017-01-01'),
    ('CIRCUIT1', 2.4,'2017-01-02'),
    ('CIRCUIT1', 2.5,'2017-01-03'),
    ('CIRCUIT1', 2.4,'2017-01-04'),
    ('CIRCUIT1', 2.4,'2017-01-05'),
    ('CIRCUIT1', 2.6,'2017-01-06'),
    ('CIRCUIT1', 2.4,'2017-01-07'),
    ('CIRCUIT1', 2.4,'2017-01-08'),
    ('CIRCUIT1', 2.4,'2017-01-09'),
    ('CIRCUIT2', 2.4,'2017-01-10'),
    ('CIRCUIT2', 2.5,'2017-01-11'),
    ('CIRCUIT2', 2.4,'2017-01-12'),
    ('CIRCUIT2', 2.4,'2017-01-13'),
    ('CIRCUIT2', 2.5,'2017-01-14'),
    ('CIRCUIT2', 2.4,'2017-01-15'),
    ('CIRCUIT2', 2.5,'2017-01-16'),
    ('CIRCUIT2', 2.4,'2017-01-17'),
    ('CIRCUIT2', 2.5,'2017-01-18'),
    ('CIRCUIT3', 2.4,'2017-01-19'),
    ('CIRCUIT3', 2.4,'2017-01-20'),
    ('CIRCUIT3', 2.8,'2017-01-21'),
    ('CIRCUIT3', 2.4,'2017-01-22'),
    ('CIRCUIT3', 2.4,'2017-01-23'),
    ('CIRCUIT3', 2.8,'2017-01-24'),
    ('CIRCUIT3', 2.4,'2017-01-25'),
    ('CIRCUIT3', 2.4,'2017-01-26'),
    ('CIRCUIT3', 2.4,'2017-01-27');

SELECT *
FROM   tabtest
WHERE  source = (SELECT a.source
                 FROM   (SELECT a.source,a.voltage,a.timestamp,Count(*) AS rn
                         FROM   tabtest a
                                JOIN tabtest b
                                  ON a.source = b.source
                                     AND a.timestamp >= b.timestamp
                         GROUP  BY a.source,a.voltage,a.timestamp) a
                        LEFT JOIN (SELECT a.source,a.voltage,a.timestamp,Count(*) AS rn
                                   FROM   tabtest a
                                          JOIN tabtest b
                                            ON a.source = b.source
                                               AND a.timestamp >= b.timestamp
                                   GROUP  BY a.source,a.voltage,a.timestamp) b
                               ON a.source = b.source
                                  AND a.rn = b.rn + 1
                 GROUP  BY a.source
                 ORDER  BY Sum(CASE WHEN a.voltage <> COALESCE(b.voltage, a.voltage) THEN 1 ELSE 0 END) DESC Limit 1)


absolute service time: 0,5 sec