Many national metrology institutes (NMIs) employ the Josephson effect for maintenance of the representation of the voltage unit. The Josephson effect voltage standard (JVS) is the most stable and reproducible representation of the volt. In smaller national laboratories the voltage standards are frequently maintained in electrochemical cells or solid-state references, so-called secondary standards, which are less expensive for establishment and maintenance. The measurement uncertainty (k=2) of the voltage standards maintained in electronic references is typically less than 1 µV/V. This can be achieved by maintenance of the group of voltage standards, whose outputs are regularly compared to the average of the whole group.