The correct fuse rating for each, unmodified, unit is detailed on the rear label - by the IEC male socket/fuse holder.
Later models have a fuse holder which holds both the active fuse and a, supplied, spare.
To protect the unit, the fuse rating needs to allow the fuse to rupture before passing current in excess of that normal and safe for the device.
This figure will be based on normal operating conditions..
However, at the instant that a transformer is first connected across the mains (unit switched on) a transient current up to 10 to 50 times larger than the rated transformer current can flow for several cycles.
The inrush current *** will depend on the transformer characteristics and the point on the power curve at which connection is made to the AC source. If the transformer core retains any residual magnetism the inrush may well be even more severe.
The fuse chosen to protect the unit needs to withstand instances of momentary, extraordinary demand while not being so highly rated as to compromise everyday protection.
The fuses are T rated – or Slow Blow.
Naim have found that fuses of the same rating but different manufacturer do not provide the same outcome when used in Naim units.
In Production and Service we use fuses from a specific series within the Littel Fuse Company range. We strongly advise you continue to use the exact same as replacements.
These should be readily available from your Naim retailer – although some will source their stock elsewhere. You should check that your Naim retailer carries a full range of spare fuses – either from Naim Salisbury or the exact same items from a Littel Fuse supplier.
Fuses of the correct rating from other manufacturers may work and should be safe. However, some Naim units make greater demands on the rating (the XPS-2 for example) and non-Littel fuses are likely to rupture – always or more often.
Repeated blowing of the correct Naim supplied fuse is an equipment fault indicator.
Repeated blowing of A.N.Other (correctly rated) fuses MAY be a fuse fault indicator.
When obtaining a replacement fuse – get at least one spare and tuck it in the spare fuse holder for emergencies.