EXEL Ltd. began offering Fortune 500 companies up to $50 million in supplemental property insurance.

The coverage will be triggered when a client sustains losses of $300 million or more from a single incident.The Hamilton, Bermuda, company would write such excess insurance at levels where primary insurance coverage ends. It already provides supplemental liability coverage to the same clients and is expanding into property underwriting at the request of its clients because such insurance has been in short supply, said Gavin Arton, EXEL's senior vice president of investor relations.

EXEL's supplemental property coverage may be offered to some clients only in cases where the single incident involves losses of $600 million or $700 million, Mr. Arton said. The policies will be written by EXEL's underwriting subsidiary, X.L. Insurance Co.

"We want to be the top layer on that book of business," Mr. Arton said.

Michael J. Kevany, EXEL's chairman and chief executive officer, said, "By building on our strong excess general and professional liability insurance relationships and broker distribution network, the company is uniquely positioned to offer this new coverage and add considerable value to the products and services now provided to our existing insureds."

EXEL has been writing excess liability insurance of up to $75 million beginning at an average level of $58 million, Mr. Arton said. It also has written some excess liability policies that start at levels as low as $25 million, he added.

Industry analysts said EXEL's strategy is wise because there is limited competition for writing insurance at such levels. One new competitor in the excess liability area is Starr Excess, also of Bermuda, which was launched earlier this year by American International Group Inc. and other investors.

ACE Ltd., another excess liability insurer in Bermuda, typically writes coverage at higher levels than either EXEL or Starr.

EXEL should be able to handle its expanded business by using existing underwriters and consultants that already have expertise in property insurance, but a senior underwriter may need to be added as demand for the product grows, Mr. Arton said.

EXEL's entry into property underwriting will allow it to spread its risk and broaden its premium base, Mr. Arton said.

"We will be getting extra premium to augment what we're already doing in the liability area," Mr. Arton said. "We know these customers quite well. We think the profit margins are potentially better in property underwriting at this level than generally for excess liability coverage."

EXEL will develop an underwriting unit within the organization specifically to handle its property insurance, Mr. Arton said.

EXEL earned $286 million in its latest fiscal year, which ended Nov. 30, 1992, compared with $267 million for the fiscal year ended 12 months earlier. It has more than $3.5 billion in assets and shareholders' equity of nearly $2 billion.