OSM_EE 13.4
Question:In EN 60950-1, there are requirements for fuses in Sub-clause 2.7.3:
2.7.3 Short-circuit backup protection
Unless appropriate short-circuit backup protection is provided, protective devices shall have
adequate breaking (rupturing) capacity to interrupt the maximum fault current (including shortcircuit
current) which can flow.
For PERMANENTLY CONNECTED EQUIPMENT or PLUGGABLE EQUIPMENT TYPE B, it is permitted for
short-circuit backup protection to be in the building installation.
For PLUGGABLE EQUIPMENT TYPE A, the building installation is considered as providing shortcircuit
backup protection.
NOTE If fuses complying with IEC 60127 are used in PRIMARY CIRCUITS, they should have high breaking capacity
(1 500 A) if the prospective short-circuit current exceeds 35 A or ten times the current rating of the fuse, whichever
is greater.
The 3 questions are concerning a Class II wall plug adaptor (a.c. mains input, d.c voltage output) that is Pluggable Equipment Type A. The fuse in the AC primary circuit, if equipped, is mostly not of the High Breaking Capacity type.
Q1:
Sub-clause 2.7.3, first paragraph does not require that protective devices (ex. fuses) in AC primary circuits of a product must have an adequate breaking capacity if appropriate short-circuit back-up protection is provided. In practice, this means that fuses or circuit-breakers in the building installation are considered to provide this protection. In normal household installations, such protection allows currents up to 16A or 20A before they break a fault current. This means that no fault protection is provided up to a power level of 230V x 16A = 3680W, roughly 4kW.
Is a building installation that only breaks a fault current if it exceeds 16A, producing a faulty power consumption of roughly 4kW, considered as an <appropriate> protection?
Q2:
Most fuses in the AC primary circuits comply with IEC60127 but are not of the High Breaking capacity type.
Hence, if a fuse is equipped in the AC primary circuit of the Pluggable equipment Type A product and this fuse complies with IEC60127, is compliance needed with the requirement of High Breaking capacity in the NOTE?
In this context, an explanation may be needed about the meaning of “prospective short-circuit current”: is this a steady state current or a higher short term current that can be well above the nominal 16A nominal fuse or circuit breaker value?
Q3:
Sub-clause 2.7.3, first paragraph, requires an “adequate breaking (rupturing) capacity”. If Q2 does indeed not require a High Breaking capacity type, what are the parameters to determine that “adequate capacity”?
Decision:
OSM agreed to the following:
A1: See 2.7.1 in common modifications
A2: It is not necessary to have a high breaking capacity or enhanced breaking capacity fuse, as long as the fuse does not explode while performing the fault conditions.
A3: The test should be done on a 16A unloaded branch circuit (for UK see SNC) and the fuse in the product shall operate.
EN 60898-1 circuit breakers type B are considered the most spread type in Norway and in several other countries.
The OSM is of the opinion that type B can be used and it is assumed that this type of circuit breaker will cover the conditions in European countries.
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