(49)It is essential for any payment service provider to be able to access the services of technical infrastructures of payment systems. Such access should, however, be subject to appropriate requirements in order to ensure integrity and stability of those systems. Each payment service provider applying for participation in a payment system should bear the risk of its own choice of system and provide proof to the payment system that its internal arrangements are sufficiently robust against all kinds of risk. Those payment systems typically include the four-party card schemes as well as major systems processing credit transfers and direct debits. In order to ensure equality of treatment throughout the Union as between the different categories of authorised payment service providers, according to the terms of their licence, it is necessary to clarify the rules concerning access to payment systems.