Even the best plan can be drained dry by getting sick. Here's how to protect yourself with disability insurance.

By Janice Revell, contributor

FORTUNE – Let's say you're a diligent retirement planner, the type who maxes out his 401(k) and IRA. Odds are, however, that you devote little thought to protecting the value of your human capital -- that is, your ability to earn and save money. Ignore it at your peril: Throughout much of your life, your earning power is far more valuable than your stock portfolio; losing it for just a couple of years could derail your retirement goals.

How vulnerable are you? Consider this: One in three employees between the ages of 35 and 65 will, at some point in their career, be out of work for three months or more because of injury or illness, according to the Society of Actuaries. Freak accidents aren't the primary culprit: 90% of all disability claims are for common illnesses and health conditions, such as back problems and cancer, according to a recent report by the Consumer Federation of America and employee benefits provider Unum. Advisers warn not to be cavalier about your ability to weather a longer-term illness. "Look at your cash flow, then put your hand over your employment income and ask yourself how well you'd do," says Paula Hogan, a Milwaukee-based financial planner. "Even among the affluent, the answer usually isn't pretty."

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That's why long-term disability insurance, which pays for income loss caused by prolonged illness and injury-related absence from work, is crucial. Unlike short-term disability insurance (a.k.a. sick leave), which typically covers your earnings for only a few months, long-term disability insurance can replace your income until you reach age 65. About one-third of American workers have access to long-term disability insurance through their employers.

But if your income is in the six figures, any work coverage you have is probably still not enough. For starters, employer-provided disability generally provides a maximum benefit equal to 60% of base salary -- the vast majority of group plans do not include bonuses, commissions, or other incentives. And most have a benefit cap, commonly in the range of about $10,000 a month. The result can be a huge income gap at the worst possible time.

The solution is to purchase an individual disability policy from an insurance company. Under a personal policy, higher-income earners can generally arrange to replace up to 70% of their total pretax income. And the payouts will be tax-free. When buying a policy, look for "own occupation" coverage, which covers you if you can't perform your current job, as opposed to policies that don't pay if you're able to work in any job at all. The annual premium should be about 2% to 3% of the income you're looking to replace until age 65. Think of it as an investment in yourself.

A former compensation consultant, Janice Revell has been writing about personal finance since 2000.

This story is from the July 12, 2012 issue of Fortune.

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