A picture can speak a thousand words. See the diagram at http://www.axa.nl/zakelijk/personeel/arbeidsong/excedent.cfm for a quick explanation! (WAO cover is just one example of the possible use of this type of insurance.)

Eurodic gives:
"NL excedentverzekering
EN excess insurance
Definition: an insurance taking effect only in the case of losses exceeding a determined amount or the amount covered by a previous or primary insurance to which it does not make contribution"

In insurance terms, 'excess' is BrE for AmE 'deductible'.

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Note added at 1 day 11 hrs 59 mins (2004-08-18 08:37:27 GMT)
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Take a look at this website: http://www.aigaviation.com/nac/aviNAC_commercial_hull_ded.ht...

Here we read that \'deductible insurance\' is additional coverage that you can take out to cover risks that would normally fall under the deductible/excess/\'eigen risico\' of the policy.

-> \"Losses within the All Risks policy deductible are often costly and never predictable. ... Many of these losses, such as bird strikes, hail storms and damage caused by third parties, are completely outside of the control of the operator. ... AIG Aviation offers a range of insurance products and services that enable you to manage the financial impact of these losses, including: Standard deductible insurance\".

Another good explanation:
-> \"Deductible insurance

Covering an airline against the uninsured first loss deductible, which the insured is required to assume under an Airline Hull All Risks Insurance policy. Deductibles are usually $500,000 for narrow-body, $750,000 for B737/A320 types and $1million for wide-body. All losses and accidents suffered by airlines result in high shock costs, which can challenge an airline\'s ability to stay in business. The LPH deductible programme covers airlines against the otherwise guaranteed exposure to significant uninsured repair cost.\"
http://www.lphpitman.co.uk/aviation.htm

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Note added at 1 day 12 hrs 9 mins (2004-08-18 08:47:21 GMT)
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Kijk ook bijvoorbeeld hier voor Nederlandstalige informatie over luchtvaart verzekeringen: http://www.bmlhypotheken.nl/main.asp?page=95 en http://www.aviabel.be/nl/corpsetrc.html