There is an example in the Redfish Spec. as follow:
If a property in a response is a reference to another property within a resource, the "URI Fragment Identifier Representation" format as specified in RFC6901 shall be used. If the property is defined as a reference property within the schema, the fragment shall reference a valid resource identifier. For example, the following fragment identifies a property at index 0 of the Fans array within the resource /redfish/v1/Chassis/MultiBladeEncl/Thermal:
Since the Redfish specification makes normative references to RFC6901, the JSON fragment encoding must follow the rules in that RFC. With regards to the array evaluation portion of RFC6901, it does state that the token must be a zero-based index into the array: "characters comprised of digits (see ABNF below; note that leading zeros are not allowed) that represent an unsigned base-10 integer value, making the new referenced value the array element with the zero-based index identified by the token".
So, using something like "#/Voltages/3V" is illegal, and also using "#/Voltages/3" as the first element in the array is also illegal.