Short answer: SSDI pays only if the person cannot do any substantial gainful work and the impairment is expected to last at least 12 months or result in death, after a 5-month waiting period. Group LTD pays under the policy’s own definition, often starting with the person’s own occupation, after an elimination period the employer chose.
The SSDI definition is strict. Under 42 U.S.C. 423(d), disability means inability to engage in any substantial gainful activity because of a medically determinable impairment expected to result in death or to last at least 12 continuous months. The person must be unable to do not only their previous work but any other work that exists in the national economy, considering age, education and work experience. Benefits begin after a waiting period of five consecutive calendar months of disability (423(c)(2)).
Group LTD is a contract. It defines disability its own way, often as inability to do one’s own occupation for an initial period. It pays a percentage of the person’s own earnings rather than a benefit based on the Social Security earnings record, and it can pay partial benefits for a return to part-time work.
SSDI brings Medicare after 24 months of entitlement (42 U.S.C. 426(b)); LTD does not provide health coverage. The two usually work together: LTD pays first and reduces its benefit once SSDI starts.