This guidance was written prior to the February 27, 1997 implementation of FDA's 
Good Guidance Practices, GGP's. It does not create or confer rights for or on any person 
and does not operate to bind FDA or the public. An alternative approach may be used if 
such approach satisfies the requirements of the applicable statute, regulations, or both. 
This guidance will be up dated in the next revision to include the standard elemnt s of GGP 's .  .  . 
DEPARTMENT OF HEALTH. EDUCATION. AND WELFARE 
PUBLIC HEALTH SERVICE 
FOOD AND  DRUG ADMINISTRATION 
ROCKVILCE. MARYLANO  20857 
e 
~ A 2R    I980 
To:  A l l  Manufacturers  and Importers of  Microwave Ovens 
Subject:  Open Door  Operation of Microwave Ovens as a Result  of 
Oven Miswiring 
Recently there have been three incidents in consumer locations in which 
the microwave oven operated with the door open due t o  oven miswiring. 
These  incidents have resulted in expensive oven retesting and recall 
programs.  It has also been learned that  several workers at one manu- 
facturing location had  accidently been  exposed to microwave radiation in 
several instances during oven testing,  due to oven miswiring that re- 
sulted in the continued operation of the oven after the daor was  opened. 
The  Bureau is greatly concerned that  future  incidents such as these 
could occur and result in the exposure of  individuals to high levels of 
microwave energy.  To  reduce the potential for such open door operation, 
due to miswiring of  the oven circuitry, the Wtreau  strongly recommends 
that all manufacturers  consider  instituting design features that will,  (1) 
preclude andagous accidental miswiring -of microwave  ovens,  or  (2) if 
miswiring occurred,  prevent the unit from operating with the door open. 
It is recommended that all manufacturers  consider: 
1.  Locating one  safety interlock on the neutral  side of the line 
and another on  the high  side of the line. 
2.  Locating the monitor  circuit as the last circuit  branch in 
parallel with the high voltage transformer,  or making the "hot" 
A.C.  line connection to the monitor  circuit the last connection prior 
t o  connection with the primary of  tbe power  transfomer. 
3.  Performing a failure mode  effects analysis on each unique 
design to identify critical connections and wires where  the ac- 
cident&  interchange would result  in electrically by-passing  all 
safety interlocks and the monitor. 
4.  Designing  critical wire terminal connect p i n t s  so as to 
mechanically  preclude accidental interchange of  these wires to 
any unsafe  configuration. 
5.  Covering the terminals ',of critical wires with insulation to 
prevent  any loose wire from being able to contact another point 
that might  result in by-passing  the safety interlock or monitor 
. 
circuit 
6.  Using  wire ties to limit the degree of movement  of  any Kire 
that  could becone disconnected.  . 
- 4 .   - 
Page  2  A l l  Manufacturers and Importers of  Microwave  Ovens 
7.  Instituting other design features that would  prevent  the mis- 
wiring of  units or prevent units that are miswired  from operating 
with the door open. 
Manufacturers must  be caref'ul  not  to introduce design changes that could 
result  in noncompliance with the Federal perforinance  standard.  It is 
requested that proposed  changes be .submitted to the Bureau for review 
prior to incorporating the changes on production models-  It may also be 
appropriate to submit prototype units to the Bureau  for evaluation. 
It  is equally important  that certain good  quality control practices be . 
observed to help prevent these undesirable conditions.  The  Bureau  is 
hereby requesting that all manufacturers  now  institute,  if  not  already 
a  part  of  their quality control and testing program,  a specific test of  . 
each unit to determine that the unit w i l l  cease operating when  the door 
is opened,  and that it cannot be restarted,  when  the door  is open.  Any 
test that provides positive indication of magnetron  shutdown upon latch 
release or door opening w i l l  be acceptable. 
The  test must  also include an attempt to restart the unit while the door 
is unlatched.  This should be attempted both with the unit programed and 
not  programed  (for operation)for digital units,  and Kith and without time 
on the timer  for nondigital units,  if the status of  these controls could 
alter the possibility of  starting with the door  open- 
We  recognize that these tests require care in order t o  prevent  the 
exposure of the operator to high levels of  microwave  energy-  In each 
case the door  should be unlatched  slowly and the operator instructed to 
terminate the test imnediately if a problem is noted.  Also  a mechanical 
stop is recommended that would  allow the door to be unlatched and opened 
slightly but  which would prevent the door  from opening t o  a position 
that would allow hzzardous levels of microwave energy t o  leak from  the 
oven. 
Regardless of  the type of  test performed  the test must  be conducted at a 
point on the production that is subsequent to any assembly,  adjustment 
or repair of the unit that could alter the oven wiring.  A l l  operators 
must  be  instructed to immediately report any findings of open  door 
operation to the proper  quality control person  so thzt the defective 
unit czn be analyzed to determine the cause of  the failure.  Manu- 
facturers should take corrective action to prevent  future incidents. 
Since tne failure could be the result of  a  defective wiring component 
that is assembled  in-house  or purchased  from a vendor,  it is recommended 
that the assemblers of  kuch  components receive additional training and 
assenbly aids that,will help prevent  defective wiring components.  Units 
of  the components  should be  inspected on a  sampling or 100% basis prior  - 
Page  3  A l l  kfanuf acturers and  Importers of  tficrowave  Ovens 
to being  sent to the production  line-  If a defective component  is found 
in a  lot,  then that lot should be rejected or 100 per  cent tested prior 
to allowing  the components to be  sent to the assembly  line.  Where 
sampling is used, lots received  after a rejected lot should be  tested 
under  a tightened  sampling  schedule until the results allow for return 
to a normal  sampling  schedule. 
Please institute any design and testing program change-s necessary to prevent 
open door operation,  The Wlreau would  welcome  other  suggestions which 
could help prevent  such incidents. 
Y  Robert G.  Britain 
Director 
Division of  ~aa&iance 
Bureau of  Radiological Health