Demo, Sql Server - rextester

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CREATE TABLE Table1
    (id int identity(1,1),[source] varchar(8), [voltage] numeric)
;
    
INSERT INTO Table1
    ([source], [voltage])
VALUES
    ('CIRCUIT1', 2.4),
    ('CIRCUIT1', 2.4),
    ('CIRCUIT1', 2.5),
    ('CIRCUIT1', 2.4),
    ('CIRCUIT1', 2.4),
    ('CIRCUIT1', 2.6),
    ('CIRCUIT1', 2.4),
    ('CIRCUIT1', 2.4),
    ('CIRCUIT1', 2.4),
    ('CIRCUIT2', 2.4),
    ('CIRCUIT2', 2.5),
    ('CIRCUIT2', 2.4),
    ('CIRCUIT2', 2.4),
    ('CIRCUIT2', 2.5),
    ('CIRCUIT2', 2.4),
    ('CIRCUIT2', 2.5),
    ('CIRCUIT2', 2.4),
    ('CIRCUIT2', 2.5),
    ('CIRCUIT3', 2.4),
    ('CIRCUIT3', 2.4),
    ('CIRCUIT3', 2.8),
    ('CIRCUIT3', 2.4),
    ('CIRCUIT3', 2.4),
    ('CIRCUIT3', 2.8),
    ('CIRCUIT3', 2.4),
    ('CIRCUIT3', 2.4),
    ('CIRCUIT3', 2.4)
;

SELECT TOP 1 WITH ties *
FROM   (SELECT *,
               Lag([voltage])OVER(partition BY [source] ORDER BY id) AS prev_val
        FROM   table1)a
ORDER  BY Sum(CASE WHEN [voltage] <> Isnull(prev_val, [voltage]) THEN 1 ELSE 0 END)
            OVER(partition BY [source]) DESC

View schema
Execution time: 0,02 sec, rows selected: 9, rows affected: 27, absolute service time: 0,17 sec