Specifications

22 Issue 5, August 1994 Acorn Enhanced Expansion Card
Acorn Enhanced Expansion Card
Safety
The current industry wide IT safety standard is IEC950
whose European ‘harmonised’ version is
EN60950 / BS7002, but your particular application may
also be within the scope of other additional standards.
The main requirements are that the equipment provide
protection against:
• the spread of fire
• hazardous voltages or energy.
The spread of fire
The Underwriters Laboratory (UL) of the USA have
devised several standard ways of testing plastic
materials for their flammability properties. The UL94 test
procedure is used within IEC950 to specify the required
flame retardant level for materials and components. The
ratings start at 94v-0 down to 94v-1, 94v-2 and finally
94HB.
Confirmation of a UL test pass will be the issue of a
‘yellow card’, a copy of which can be obtained from your
supplier, for either the plastic material itself or the
component.
IEC950 specifies that PCBs will have a minimum
flame-retardancy rating of UL94v-1 and that any
components mounted on the PCB meets the lower
standard of UL94v-2.
The choice and layout of components should also seek
to prevent the spread of fire across the PCB and within
the computer.
The computer external enclosure forms a fire barrier and
as such its material must meet the higher standard of
UL94v-1. As the expansion card rear panel will be part
of the fire enclosure the panel itself must meet this
standard; metal is acceptable.
If you fit a large plastic connector into this rear panel the
connector will also have to be UL94v-1.
Hazardous voltages or energy
The computer PSU is designed to provide only SELV
(Safety Extra Low Voltage) to the computer. This means
that the PCB supply voltages have two independent
means of protection against hazardous voltages, thus
ensuring that even under single fault conditions the
voltage on the computer PCB and interfaces will be safe.
Within IEC950 hazardous voltages are those greater
than 42.4V peak or 60V DC.
Physical access by a user to any hazardous voltages
must be prevented by a physical barrier in which no hole
is greater than 5 mm in diameter, and no slot is wider
than 1mm, regardless of its length. For full details, see
Section 4.3.16 of IEC950.
Expansion cards must be designed in such a way that
they do not introduce either a reduction in fire protection
or of voltage isolation into the host computer. Expansion
card designers should obtain a copy of the standard and
if necessary seek further clarification by consulting a
reputable test facility such as BSI.
With the EEC Directive on General Product Liability
manufacturers and importers within the Community are
subject to strict liability. This existing law removes the
plaintiff's requirement to prove that a product was
‘defective’ when seeking damages. The possession of
an EN certification may not be an adequate defence and
therefore manufacturers need to be aware that there
may be safety aspects of their product which are not
covered by the standards.
Safety testing: Electricity at work
regulations, 1989
U.K. legislation now requires that portable electrical
equipment be tested regularly, usually annually, for
safety. Be aware that this testing may be carried out by
a variety of people of varying technical competence and
experience.
Class 1, earthed, equipment will be tested by the
application of a high current, low voltage, source of
4-25 Amps, between any exposed metalwork and the
earth pin in the mains plug. The presence of your
expansion card in a computer will make it liable for
testing.
You should therefore consider whether to provide
information in your documentation to either the user or
the dealer on how your card should be tested.
Important Note: Within some Risc PCs, the EMC
coating provides a functional earth to the Eurocard style
expansion cards. The EMC gasket on the rear panel of
the card contacts the coating at a number of points.
These contact points are not capable of carrying the
protective earth test current of 25 Amps. If a card is
subjected to this high current the coating at the contact
points will burn away and you will lose the functional /
EMC earthing of the expansion card rear panel. This will
not reduce the safety of the equipment but will
compromise the EMC performance, and possibly the
functionality, of the card. You should therefore ensure
that any expansion card documentation mentions this
point.
EMC design
Acorn's computers are designed to meet current EMC
requirements. Expansion cards should be designed
such that the existing EMC performance of the computer
is not reduced. The following points should be borne in
mind.
• To provide the Risc PC with good EMC performance
all expansion cards should be fitted with an EMC
gasket. The blank version of the EMC gasket is