WASHINGTON, May 18 (Reuters) - Credit rating agencies would have to disclose more details about their ratings process and keep close tabs on employees who leave to work for issuers, under a plan proposed by U.S. securities regulators on Wednesday.

The Securities and Exchange Commission voted unanimously to seek public comment on over 500 pages of proposals, the latest series of measures required under the Dodd-Frank financial oversight law.

Additional requirements would be imposed on credit-rating firms including Moody's Corp, McGraw-Hill Cos Inc's Standard and Poor's and Fimalac SA's Fitch Ratings, in an effort to hold them more accountable for their performance after they gave top ratings to mortgage securities that collapsed during the financial crisis.

The proposals include establishing firewalls between the sales and analytical sides of the credit-rating business, and holding more accountable the third-party firms that assess the quality of asset-backed securities.

Still, the lengthy list of measures is expected to only lightly touch industry practices, as the big firms already have similar procedures in place to address conflicts of interest.

The measures expected to have the biggest impact on the industry are those that aim to reveal the methods used to conduct each rating, and the statistical data reflecting how well raters perform over time.

The SEC had previously adopted rules in both these areas, but was required by Dodd-Frank to build on them so that the investing public can better gauge the performance of credit ratings.

One of Wednesday's measures would standardize statistics on the performance of a rating and the default of the product so that investors can see which firms downgraded securities first or were more accurate in their assessment.

Another measure would require additional disclosures on the methods used to calculate each individual rating. Methodologies would also have to be approved by the board of the ratings firm, and any errors or material changes would need to be promptly disclosed to the public.

In the area of conflicts of interest, a separation would be required between the sales and marketing division and the division that establishes and monitors ratings. Any firm that violates this provision could be subject to penalties, although smaller companies with fewer resources could seek an exemption from this requirement.

In addition, the proposals would implement a "look back" provision under which credit-raters would need to establish procedures to monitor when employees leave the company to work for an issuer that received a rating within a one-year time frame.

The ratings firms would be responsible for determining if any conflicts exist and whether they need to reissue the rating.

(Reporting by Sarah N. Lynch)