60 V AC behind DC step-down
Assuming you have a 120VAC power feed into your premises, half that voltage "can" be seen when you haven't got a proper earth: -







simulate this circuit &ndash; Schematic created using CircuitLab

A lot of power supplies have EMI reduction circuits on input and output and these shunt switching noise to the earth line. When you don't have the earth connected properly the 120VAC supply voltage forms a capacitive potential divider with C1 and C2 - the net result is 60V AC on the earth wire and this AC inflicts itself on the DC output via C3 and C4.

EDIT - The OP has said that the incoming AC supply is 240VAC so now to justify 60VAC on the output, it is necessary to look at how this measurement is made. The same basic principle stands - the lack of earth can cause an infliction of input AC power to the output wires via the EMI protection components.