Q:

Are there general accuracy requirements for aircraft power supplies?

Let's say I have some LRU device that requires some DC voltage to be powered on. Are there any general requirements in aviation/aerospace that say how accurate that supply voltage must be in order to be allowed on a plane?
Update:
To be more specific, consider an LRU that takes 28V on the input and provides it to a DC-DC converter. I would like to know how accurate that 28V being provided, needs to be.

A:

Short answer:

Operational voltages on the 28 V bus may range from 22 V to 30 V. 
Abnormal voltages on the 28 V bus may occur from 0 V to 60 V.

The detailed requirements depend on the manufacturer, the type of plane and the jurisdiction.
USA: MIL-STD-704
US military standard MIL-STD-704(F) Aircraft Electrical Power Characteristics defines electrical properties of military aircraft equipment. For instance the 28 V DC bus is specified as:

A similar standard should exist for commercial aircraft, unless they just apply MIL-STD-704 as well.
Europe: ISO 1540
In Europe there is ISO 1540:2006 Aerospace -- Characteristics of aircraft electrical systems. This standard differentiates by the actual power source:

Category A: TRU (transformer)
Category B: engine driven DC generator
Category R: regulated DC power from active converter

Category A and B are specified identically for their DC characteristics. Category B is the worst case for AC ripples. Category R is the best case, as it is already regulated. Nevertheless, connected equipment needs to cope with the worst case, that is category B (engine driven generator).
ISO 1540 specifies the 28 V DC bus characteristics (summarized) as:

28 V DC bus - Normal operation: 22 V - 30 V
28 V DC bus - abnormal/onset: 0 V - 60 V

