Kumar's 50.16 seconds effort eclipsed the previous meet record of 50.26 seconds set by Joseph Abraham in 2009.
Services' athlete Ramachandran was second while Aftab Alam of Uttar Pradesh was third at the Nehru Stadium here.
The men's high jump too saw a new meet record being set by both Siddarth Yadav (Railways) and V Bharathi (Services), clearing 2.23 metres, putting in shade the old mark of 2.22m set by Tejaswin Shankar last year.
Similarly in the pole vault, the top three achieved a height of 5m but J Preeth of Railways took the gold on countback.
K Ganapathi of Services walked his way to a gold in the 20km walk while his teammate Sandeep Kumar finished second followed by Manish Rawat of All India Police Sports Board.
Ganapathi, who participated in last month's World Athletics Championship in London, was a comfortable winner in the race held in the morning under cloudy skies. He, in fact, posted a better time than he did in the Worlds by finishing in 1:27:33.
In the women's 400m hurdles, ONGC's Jauna Murmu romped home in 58:25 seconds, way ahead of M Arpitha (Railways) and Punjab's Veerpal Kaur.
There were gold medals for Karnataka's D Aishwarya in triple jump and for Anu Rani of Railways in the javelin throw in which Uttar Pradesh's national record holder Annu Rani, who participated in the London World Championships, did not start.
Results: Men: 400m Hurdles: 1. Santhosh Kumar (Tamil Nadu) 50:16 (NMR), 2. Ramachandran (Services) 51:02, 3. Aftab Alam (Uttar Pradesh) 51:33.
High Jump: 1. Siddarth Yadav (Railways) 2.23m, 2. V Bharathi (Services) 2.23m, 3. KS Anil (Services) 2.21m.
3,000 metres Steeplechase: 1. Sable Avinash (Services) 8:39.81; 2. Shanker Lal Swami (Rajasthan) 8:56.68, 3. Jaiveer Singh (Services) 8:58.99.
Discus Throw: 1. Dharam Raj Yadav (Services) 55.08m, 2. Prabhjot Singh (All India Police Sports Board) 53.69m, 3. Gagandeep Singh (Punjab) 52.53m.
Pole Vault: 1. J Preeth (Railways) 5.00m, 2. Siva (Services) 5.00m, 3. Bimin K P (Railways) 5.00m.
Women: 400m Hurdles: 1. Jauna Murmu (ONGC) 58:25, 2. M Arpitha (Railways) 59:97, 3. Veerpal Kaur (Punjab) 1:00.60.
Triple Jump: 1. D Aishwarya (Karnataka) 13.03m, 2. Rintu Mathew (Railways) 12.86m, 3. Joyline Mural Lobo (LIC) 12.84m. 3000 metres Steeplechase: 1. Chinta Yadav (Railways) 9:49.23, 2. Chhavi Yadav (Railways) 10:16.34, 3. Khushbu Gupta (UP) 10:44.40.
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