Hermann Spengel was a member of the short-lived South African Society of Electrical Engineers, which flourished in Johannesburg from May 1897 to the outbreak of the Anglo-Boer War late in 1899. In August 1899 he read a paper at a meeting of the society on "The apparatus used for the control and distribution of H.T. [high tension] currents". He became a member of the Chemical and Metallurgical Society of South Africa in 1896 and was still a member of the renamed Chemical, Metallurgical and Mining Society of South Africa in 1905. In August 1897 he joined the South African Association of Engineers and Architects (which was renamed the South African Association of Engineers in 1898). In October 1902, soon after the end of the war, he read a substantial paper on "Electric winding problems" before the members of this society in Johannesburg. The paper was published as a pamphlet (Johannesburg, 1902, 33p).
In 1903 Spengel served on the Technical Education Commission, appointed by the lieutenant-governor of the Transvaal Colony to enquire into the foundation of a technical institution as part of a teaching university. By 1903 he was a member of the South African Association for the Advancement of Science and was still a member in 1910. In 1905 he worked as a consulting electrical engineer for Siemens, Ltd., Johannesburg, but the next year he resided in Brakpan. In the General directory of South Africa for 1908 (but not in subsequent years) he was listed as manager of The Rand Central Electric Works in Market Street, Johannesburg. He still lived in Johannesburg in 1910.