RADFORD LABPACKS
The Radford Labpack laboratory bench power pack appeared in 1959 and was one of the first
portable low-voltage power supplies for school use. Many school physics labs were
equipped with Labpacks which provided a wide range of ac and dc outputs particularly
suitable for work with thermionic valves. During late 1995 and early 1996 a number of
accidents were reported to CLEAPSS in which pupils and staff received serious electric
shocks from Labpacks. CLEAPSS had already recommended that the 300V dc output be
disabled or key locked, since it could deliver dangerously high currents. However, some of
the shocks reported were received from the low-voltage outputs. This was due to
deterioration or damage to the internal insulation caused by overloading which may not
have been detected by the standard electrical portable-appliance test. Further investigation
has led us to conclude that several aspects of the original design of Labpacks would not
satisfy the safety requirements for the design of school laboratory power supplies today.
1.
CLEAPSS recommendations
1.1
If at all possible, schools should take Radford Labpacks out of use now and replace
them.
1.2
If a Labpack must continue in use, it needs a thorough internal inspection, a special
electrical test and may need immediate modification to meet higher safety standards.
1.3
The regular portable appliance test must now include internal inspection and a more
severe electrical test.
2.
Replacement
The Labpack has proved to be a very versatile power supply, different variants providing all
or some of the following outputs.
AC
6.3 V
3A
(fixed)
AC
12 V
8A
(fixed)
AC
0 to 22 V
8A
(variable in 0.2 V steps)
DC
0 to 22 V
8A
(variable in 0.2 V steps)
HT DC
300 V
150 mA
(fixed)
It is understandable that some science staff are reluctant to dispose of Labpacks. Modern
power supplies are rarely so versatile. However, for most school laboratory activities,
particularly day-to-day experiments, a simpler low-voltage power supply is adequate and
preferable. Modern power packs have more straight-forward controls and weigh considerably less. Schools now intending to replace Labpacks will find the CLEAPSS guide Generalpurpose low-voltage power supplies, R59a, very useful. Low-voltage power supplies cost
between £80 and £180 each, depending on facilities. Where an HT supply is still required,
usually for fine beam tube or thermionic investigations in A-level Physics, most schools
PS29 JRE 8/97
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© CLEAPSS , Brunel University, Uxbridge UB8 3PH
Tel: 01895 251496; Fax: 01895 814372; E-mail: science@cleapss.org.uk; Web site: www.cleapss.org.uk
would only need one unit. HT supplies should include a 6.3 V output for valve heater
filaments. Prices from the major education suppliers range from £120 to £360, so it is worth
shopping around.
We believe that money must be spent on replacing Radford Labpacks if safety is not to be
compromised. Power supplies are essential and often daily-used pieces of equipment in
school laboratories. Our investigations have shown that a very large number of variants of
the Radford Labpack left the factory; even those with identical model numbers had different
features. Some Labpacks we have seen have been repaired and/or modified several times by
science staff, technicians or repair companies. The quality of the repairs and modifications is
variable. As a result, it is hard to find two completely identical Labpacks, even within the
same school. Some are intrinsically safe, but others have fallen well short of basic standards.
This ad hoc situation is unacceptable.
3.
Continued use
If, due to financial limitations, a school is unable to replace its Radford Labpacks immediately, a phased replacement is suggested with as short a time scale as possible. Those
Labpacks which are to continue to be used must be inspected, tested and modified according to sections 4 and 5 below. The extensive nature of the modifications and the requirement
for specialised components means that modifications should not be undertaken by school
staff, but by an electrical repairer with proven experience of Radford Labpacks. This is likely
to cost up to £50 per Labpack. At this time we are only able to recommend one repairer (see
section 6). However, we hope to approve at least one other and would welcome information
to enable us to locate more.
4.
Inspection and test
Where it is decided that a Radford Labpack will continue to be used, the following
inspection and test procedures must be carried out.
4.1
The presence of the modifications listed in section 5 below must be confirmed.
4.2
A thorough, internal inspection must look for signs of overheating of components or
wiring, particularly where wires are laced together.
4.3
A normal visual and mechanical portable-appliance test must be carried out.
4.4
A more stringent class 1 electrical test must be carried out by connecting all the
outputs together to an earth point before using a standard earth-bond and insulation
tester. The insulation resistance should be greater than 1 MΩ.
Subsequently, the Labpack should be inspected and tested every term following the
procedures in paragraphs 4.2, 4.3 and 4.4.
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© CLEAPSS , Brunel University, Uxbridge UB8 3PH
Tel: 01895 251496; Fax: 01895 814372; E-mail: science@cleapss.org.uk; Web site: www.cleapss.org.uk
5.
Modifications
Each Labpack which remains in use must be checked and then modified where necessary by
a competent electrical repairer with experience of Radford Labpacks as follows.
5.1
Mains plug
A 3 A fuse must be fitted in the mains plug.
5.2
Cable entry to Labpack
Either (if fitted), remove miniature mains socket and hard wire a mains lead to the Labpack
with a strain-relief bush,
or if the mains lead enters via a grommet, replace the grommet with a strain-relief bush.
5.3
Internal wiring
Inspect for signs of damage or charring. Replace any wires showing signs of damage.
5.4
Mains fuse holder, front panel
If removable without tools, replace with flush holder having screwdriver slot removal only.
5.5
Mains indicator (neon)
If connected between live and earth, reconnect between live and neutral.
5.6
Mains voltage selector
Remove voltage selector and hard wire taps on transformer primary to give fixed input
voltage. The nominal mains voltage in the UK is now 230 V.
5.7
Transformer
Inspect visually for signs of burning or severe overheating. Very occasionally, the
transformer needs to be replaced.
5.8
AC 6.3 V output
Check wires back to transformer for charring or burning of nearby wires (especially those
carrying mains power). Either disable by cutting off wire back to transformer or Replace
wires with thicker gauge and install self-resetting thermal-overload circuit breaker.
5.9
AC 12 V output
Check wires to transformer for charring or burning. Replace wires if necessary. Install selfresetting thermal-overload circuit breaker.
5.10 AC and DC 0 to 22 V outputs
These outputs are both protected by a single circuit breaker. Check operation. Repair or
replace as necessary.
5.11 HT DC 300 V output
Either disable by cutting off wire to within a few cm of transformer and putting insulators on
end of wire or add a key switch to prevent unauthorised use of this output.
5.12 300 V fuse holder, front panel
If removable without tools, replace with flush holder having screwdriver slot removal only.
5.13 Blanking plugs
Plastic blanking plugs must be fitted with Loctite or similar adhesive to all exterior holes
created in the Labpack by any modifications.
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© CLEAPSS , Brunel University, Uxbridge UB8 3PH
Tel: 01895 251496; Fax: 01895 814372; E-mail: science@cleapss.org.uk; Web site: www.cleapss.org.uk
6. Recommended repair company
Harding Scientific Instruments
163 West Malvern Road
Malvern
Worcs
WR14 4AY
Tel:
Fax:
01684 575974
01684 893960
PS29 JRE 8/97
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®
© CLEAPSS , Brunel University, Uxbridge UB8 3PH
Tel: 01895 251496; Fax: 01895 814372; E-mail: science@cleapss.org.uk; Web site: www.cleapss.org.uk