E. C. Ernst Inc., a major electrical construction company in Washington, announced plans yesterday to acquire a large private company engaged in the same business - L. K. Comstock & Co. Inc., of Danbury, Conn.

In another potential acquisition involving a Washington area firm, Martin Marietta Corp. revealed that it has been approached by representatives of Airco Inc., about the possibility of being purchased by the diversified Bethesda aerospace, aluminium and construction materials company.

Under an agreement in principle between Ernst and Comstock, the Connecticut firm would become a wholly-owned subsidiary of the D.C. company in an exchange of stock valued at about $7.5 million. Ernst will buy all Comstock shares in exchange for 244,233 shares of common, 20,0830 shares of Series A 6 per cent cumulative preferred and 26,169 and shares of Series B 6 per cent convertible preferred stock in Ernst.

Comstock headed by Chairman Charles L. Scharfe Jr., had revenues last year of $107 million from electrical construction and engineering operations in areas of the United States from which Ernst derives "only a small percentage of its total business," an announce said.

The Airco approach to Martin Marietta is less cleay cut. Airco, Montvale, N.J. based producer of industrial gases, is seeking to thwart attempts by BOC International Ltd. of London, to gainownership of the American firm. BOC also produces industrial gases.

Martin Marietta said only that it understands it is one of a "number of companies" that have been approched by Airco. No decisions have been made by Martin management, the Bethesda firm said.

Airco said Wednesday it is "presently engaged in preliminary discussion" with another company about its takeover. BOC has offered to buy Airco shares at $43 apiece, but Airco management has asserted that the price is inadequate.

Earlier this year, BOC acquired 1.8 million shares of the U.S. firm, bringging its ownership to 49 percent in an acquisition bid that is the subject of legal challenges. BOC acquired a 34 percent interest in Airco in 1973.

In another takeover development late yesterday, Heilig-Meyers Co. of Richmond, said it is engaged in preliminary talks regarding possible acquisition of the firm by an undisclosed potential buyer. Heilig-Meyers, a home furnishings retailer, said noagreement has been reached.