Equipment for testing of low voltage fuses

TECHNOLOGICAL

UNIVERSITY EINDHOVEN

Department of Electrical Engineering

Laboratory for high voltages and high currents.

EQUIPMENT FOR TESTING OF LOW

VOLTAGE FUSES.

by

Ir. L. Verm1j

L.A.H. Wilmes

september 1967.

EH-6?-R2.

1 •

1. Introduction.

The equipment described in this report has been designed and built in this laboratory for purposes of investigating melting wires. However, jt is also suited to test low voltage fuses.

The apparatus consists of the following main parts: (soe fig. 1)

1.1. A high-current, low voltage transformer.

1.2. A 1000 Volts, 10000/uF capacitor battery with a charging device.

1.3.

A selective circuit c~o5ing Bwitch with which the teat circuit can be closed at any desired instant.

1.4. A timing device for exact control of the test cyclus.

The closing switch 81 is a "Pameco" (a trade name of Hazemeyer) circuit breaker, which has beeu somewhat modified for this purpose. (sec fig'. 2) The transformer T1, the capacitor battery C or any other suitable

voltage source may be Hsed as a power supply.

The capacitor battery can be charged up to 1000 Volts via the transformer

T2~ rectifier G1 and chargi!lg resistance R~.

T.he object to be testod is connected to the terminals 12 and 13. Sc is a solenoId operated low vo:tage circui.t breaker.

The operational. sequence as \1011 as oscilloscope iriggering is controlled

by the timing dovice. Circuit breaker S2 opens at a certain, preselected instant after switch S1 has closed the test circuite

The closing SNitch S1 must be reset by hand before a new op~rational

sequence can be started.

In the following the operation of the main parts of the t~sting device

is e-xplain~d.

2. The cjxcuit ~~osi~~ switch S1.

The taree-pole Pameco switch used for this purpose is suitable for a nominal current of '+00 amps. It haa B closing current rating of 17 kAeff. Contact rebound is negligible with thlS switch due to the construction of the contacts (parallel knife-c_ntRcts).

2.

';/hen turning the operating handle from the "u:'F" to the "ON" position. the closing spring is charged. In the commercial design the spring is :-eleased whee the

"ON"

position :is reached. We have modified the closing mechanism of the switch such that in the liON" position of the operating handle the closing spring does not release. This feature is obtained by means of an ~lectromechanicallatch. A mechanical interlock ensures that the operating spring remains in its fully charged position. Closing the switch can now be effected by energizing the

electro-mechanical latch, as a consequence of which the spring is released. The latch is operated by a 24 Volts DC solenoid. In our case it is energized by discharging a 80,uF~ 500 Volts capacitor. This forces a relatively large current pulse of short duration through the coil.

As a consequence, the closing time of the switch is highly reproducible. This feature. as well as neeligible contac~ rebound make this switch suitable as B circuit closing d~¥ico for test purposes.

The schematic diagram of fig.3 shows hov energizing of the latch

solenoid is effected. ~his circ~it is closed by igniting the thyratron by a voltage puloc gen~rated by the timing device.

The inductance of the solenoid in fully open position is approximately

O~~2 Henryo whereas t~0 ohmic resistanc~ is

35

ohms.

Ttw !:.1preacl in the closing t.ime of the 6vJitch could be improved with a solenoid of lower self~inductance.

Fig.

4

shows an oscillogram of the current as a function of time through the coil of the solenoid.

This apparatus consits of the following units (see fig.5).

3.1. ~ trigg~r unit (Schmitt-trigger).

This qnit generates voltage pulHes at instances accurately defined relative to every other zero of tte 5C cJcl~B driving voltage. The number of pulses is equal to t~1a m:.mber of complete cycles o~ tte

drivir~ voltage.

The auo"le mention~·d pulses are u':led as synchronisation pulses, which drive a number of dela.y units once the start command is given.

3.

3.2. A G8t~ circuit consisting of a bistable multivibrator which can be set by th~ sync-pulses and r?s~t by a pulse generated via the start push-button. If no reset pulse i~ biven, the multiv~bratordoes not produce an outpl-"~. sisnaL If, however,the multi'!:Lbrator is reset by prasGing tle start push-button, the first synchronisation pulse will set the IDultivibrator again and hence an output pulse o~ the

roulti~ibrator is produced.

3.3.

~iNer-units.

Each timer unit includes a monostable multivibrator which is driven

by the output signal of the gate circuit described in 3.2. The delay ti:De of

n.t'!

monost&ble llIv.ltivibr;:,.tor is determined by its time constant ~=Re. 3y varying the capaciJ;or in steps the following time '!elay

ranges can De obt a.iced: 100/US0C ~ 1 msec; 1 msoc - 10 msec;

10 raf;~C - 100 rnsec; 1~0 UlGOC = ~ sec; a;;;,d I S9C ~ 10 sec. Witt,ill

eac~ rnnge th~ delay time can be adjuBted witl & te~ t~rn precision pote~tiometer (h~lipot) providsd ~itn a high-accuracy dial.

At the ~nstC'_nt at \'Jh5..cil the ",,"'m(Js\:,&bl~ multivihx-ator is rOGct s. puls~

iE:! g~.iA~X'.;.,ted. Th.i.s pulse clri\Y~;s e, puJ_5e-shapv~ consisting of 8.

~()r.ofitable multivtbrator with R f-i:ll:ed t:'.lile-,:ol.1stai:t of 1 illrJ0Cq ttUG

producing a Mquar~-vaY~ r~ls0 of 1 M&~C dU~8t19n. This 5q~ere-~~vo

~~lse is del~v~red io an ~mitt~r-follocrerin order to oetain a low

1.mpco.anca o~_d:.puL The 1'.imiuES tlevice includes three timer-units ~,hich

are e6s~ntiall~ identical. One of them~ provided for tripping the circuit oreak~r S2. comprises a p~ls6 shapor with an outp~t pula~ of 1 sec time 6~ratio~, cecause 1 msac i~ toe short to drive home tho trip solenoid of the circuit breaker.

The square wawe output pulses of the timing device have a rise time of 50 nanosec, a pulse duration of 1 maec, reap. 1 Bec and an amplitude of 20 Volt6~ The deviation from the preselected time delay is

appToxiMately O,~)/u~ec.

Fig.

5

shows the block diagram, wter~as the circ~it diagram of the timing

devico i6 shown in an app~nclix to ':l:is r~port ~

In fig. {) the pulse options provide~ by the above <1escribed units. are shown.

~itn the set-up described in the preceding a test circuit c~n be closed within a predicted time interval of approximately 100/usec. This agrees with

I

This total spr~ad in closing ti~e is determined, mainly by the closing switch S~. while relatively speaking the 6prea~ produced by the electronic device is negligible.

This reo~lt is obtained witt a simple and rather inexpe~6ive

set-up, ~sin~ a commercial switch as a circuit ~lcser. The modifications

whic~l were necessary t.o adapt this switch to this purpose are minor

and very simple to realise. It is however quite possible to improve this device in several respects. It can therefore be expected that the total spread can be reduce~ considerably.

The total spread of 100/usec permits recording of transient phenomena with an oscilloscope using a sweep time of 50 and even 20/usec/div. This is sufficient for the purposes for which this device has been designed.

In the oscillogram of fig.

7

the current following closing of the circuit with the above described device is shown. The instant of closing haQ been changed in steps of 1 msec.

Fig.

8

shows some of;ci.llcgrarns of fusing miniature fuses of 2 Amperes

~ating. The closing angle amounts to 300

in all these oscillogramo. The preopecti'ie currant is abo'At ~OO Amperes, thl3 sourc~ Iloltage is 220 Volts,

5C

H~rt~. These oBcillcgrams have been made using a generator as ~ voltage sourCe.

From fig.

8

it can be seen that fusing php.nomena can be ob6er~ed ~ith sweep times as low as 20/usec/div.

In

lEe

publication 127 (miniature fuses) it is recommended that the test circuit aho~ld be closed 30 degrees ~ 10 degrees after a natural ,'oltage zero.

This ~ide tolerance in closing angle may be the course of considerable

differences iu test results.

In t:':.::.s report it is GLc~n' that u .J1UC~l e.-r.aller toleral.\ce is realisable

with r£ither siL:'ple a~:d inexpeI:l3ivf; :Lealls. As a conseq\:.once much li'I.ore

r<,producip.-le r<e'e~)l\ 6 of fu~ te~ting can be t1x:r-ec~~d. Mor-eoYer the

improv~d til·Un

_e

0: "the ttl'6t seq\~eace operlE; the poasibility to stud,)"

"1'E! rh.::noc;e!'la co:onected ...ith the interruption o! :3-n electric current by a fU6E\ in more detail.

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Fig.

3.

Circuit for energizing closing coil of Pamsco switch 51.

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Fig.4. Oscillogram of current through closing coil.

Fig.? Current wave forms resulting from different closing angles relative to the voltage.

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Fig.B. Current and voltage traces of the interruption of a current by a fuse taken with different sweep times around instant of fusing.

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