Nondestructive test predicts the time required for current limiters to blow /open the circuit/ when subjected to a given overload. The test method is based on an empirical relationship between the voltage rise across a current limiter for a fixed time interval and the time to blow

Swartz, G. A.

English

NASA Technical Reports Server

October 1968	 Brief 68-10364 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Nondestructive Test Determines
Overload Destruction Characteristics of Current Limiter Fuses 
A nondestructive test method has been devised to 
predict the clear time of electrical current limiters, 
that is the time required for the current limiters to 
blow (open the circuit) when subjected to a given 
overload (e.g.,two, four, or six times the rated current 
of the limiters in this application). This method pro-
vides a reliable means for predicting the clear time 
of individual current limiters under overload condi-
tions and for quality control of production-lot quanti-
ties of these units. Standard nondestructive electrical 
measurements have not been capable of accurately 
predicting the clear times of overload current limiters. 
In one destructive method commonly used, one or 
more units from each production lot are subjected 
to actual overloads for the time required to open the 
circuit. This test is unreliable because it not only de-
stroys the current limiters sampled, but also assumes 
that all of the units in each production lot have the 
same characteristics as the samples tested to destruc-
tion. 
The new test method is based on an empirical rela-
tionship between the voltage rise across a current 
limiter for a fixed time interval (and fixed current 
overload) and the time to blow. The tester consists of 
crystal controlled precise time bases for controlling 
square wave current pulses. The limiter is subjected 
to these square wave current pulses of controlled time 
duration of length such that clearing of the fuse will 
not result. This test will insure that the limiter will 
withstand transients without clearing. Voltage wave 
forms across the limiter are monitored during the 
test with an oscilloscope. 
In order to determine the relationship of the clear 
time to the voltage rise characteristic, a representa-
tive number of current limiters are each subjected to a 
pulse corresponding to the specified overload condi-
tion, and the resulting voltage rise characteristics are 
measured from the oscillograph. The duration of the 
pulse applied to each of the limiter units in this (first) 
stage of the test must be sufficiently short to avoid 
blowing or damaging the units. The same limiters are 
then each subjected to a voltage pulse of the same 
height (corresponding to the same overload condition) 
as in the previous measurement, but for a sufficient 
time to blow the units (to open the circuit). The 
resultant measured clear times values are then plotted 
against the measured pulse heights to obtain a least 
square-fit regression line. Any desired number of cur-
rent limiters of the same type (from the same lot) 
may then be nondestructively tested by measuring the 
pulse height of each unit as in the first stage. The 
measured pulse height for each unit is then used to 
determine the corresponding clear time from the re-
gression line. 
Note: 
This nondestructive test method may be applicable 
to other electrical components exhibiting dynamic 
characteristics which are power-limited. 
Patent status: 
Title to this invention has been waived under the 
provisions of National Aeronautics and Space Act [42 
U.S.C. 2457(f)], to the Electra-Midland Corpora 
tion, Independence, Kansas.
Source: G. A. Swartz
of Electra-Midland Corporation
under contract to
Goddard Space Flight Center
(XGS-08566)
Category 01 
This document was prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States	 will be free from privately owned rights.
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Nondestructive test determines overload destruction characteristics of current limiter fuses

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