The experiments that model either external or internal maintenance workers are more realistic and only include non-ideal maintenance. When using external maintenance workers, a response time is included for the maintenance workers to answer a call for CM activities. Nowadays, many companies outsource their maintenance activities and sign a service contract either with the equipment manufacturer or with third parties. Response time is the subject of one of the main clauses in these contracts. The shorter the response time to maintenance calls, the higher the service cost. This cost dramatically increases when the response time is required to be immediate or very short. Hence, plants try to find a balance between downtime cost and the cost due to short response times.