Introduction to Fuzes

GENERAL: The development of electrical time and impact fuzes has been carried on in Germany since 1926. The greater part of the work has been done by the Rheinmetall Borsig organization. All the work in this line was under the direction of Herr H. Rhulemann.
    The original object of the development was to produce and electrical time fuze for projectiles, which could be set at the instant of firing. This part of the development lagged in the early stages, and the development of electrical bomb fuzes was begun. this work was very successful and the electrical bomb fuze was adopted by the Luftwaffe in 1937. It is estimated that about 1 million time fuzes and 20 million impact fuzes were used since their adoption.

CONSTRUCTION: The construction of electrical bomb fuzes has remained generally the same since they were first adopted in 1937. For the purposes of this document, construction willbe divided into case and internal construction.

    A. Case Construction. The fuze cases for all electrical bomb fuzes have maintained, generally, the same external appearance even though there are six different types of construction. The illustration F1 shows the six types of construction. All types of construction use aluminum for the case except type 5 which has a thin sheet steel case.

    The raised portion on top of the fuze is called the fuze head or boss. One or two plungers, depending on the type of fuze, are located in the fuze head and are insulated by the black ebonite collars. The side of the fuze head is grooved to accomodate the charging head from the electrical charging unit in the aircraft. The flat portion surrounding the head is referred to as the fuze shoulder. On the side of the fuze shoulder and in line with the two plungers is a small locating pin which serves to keep the fuze in the proper position in the fuze pocket. The female threads at the base of the fuze are there to receive the standard explosive gaine.
    B. Internal Circuit Construction. The internal construction of the typical electrical bomb fuze can be divided into two sections: the upper section, and the lower section.

Illustration F4 below shows the various types of trembler switches found in German fuzes. The upper section is called the switch block. It is a polystyrene moulding which has been machined to take the various plunger contacts, the trembler switches, and in some cases the long delay igniter bridge.

    The lower section contains the storage and firing condensers, the resistances, and the instantaneous and short-delay igniters. These parts are held in place by the black bitumen calking substance.
    The condensers are constructed of metal foil strips seperated by wax paper and all wound on one cardboard cylinder. The carbon resistance normally fit into this cylinder. It may also contain the long delay cold cathode tube which is true for the E1 AZ (9).
    The igniter block fits into the bottom of the fuze and contains the black powder flash pellet, three holes leading from the pellet to the igniter bridges, and the short delay train.

ELECTRICAL PRINCIPLES: To illustrate the principle of the three circuit electrical fuze.
    A. Charging Circuit Operation. The charging circuit in the plane is of special interest. As the bomb is loaded in the aircraft, the charging head is clamped on the fuze head. The charging pins contact the plungers and depress them so that they make electrical contact with the storage condensers. The two charging pins are connected to the sliding contacts located in the charging arm. These contacts close when the bomb has fallen from one to three inches from the rack. This prevents charging of the fuze while the bomb is still in the aircraft. The two sliding contacts are conected to the positive terminal of the 240V battery. The B plunger circuit is conncted directly, while the A plunger circuit is connected through a "ZSK or selector switch. This switch has two positions: open (MV), with delay; and closed (OV), without delay. The battery is tapped at 240 volts and 150 volts. These two leads run to the voltage switch. The voltage switch can be set at 150 volts for level bombing or 240 volts for dive boming. It cannot be used to open the circuit. The voltage switch is connected to the master switch which is used to jettison the bombs. Suppodedly it is closed only when the aircraft is over enemy territory. The master switch is connected to the charging head which contacts the fuz head and completes the electrical circuit throughout the fuze body to the storage condensers.
    B. Fuze Circuit Operation. For the purpose of illustration we shall assign arming times for the three sircuits as follows: Instantaneous circuit, 8 second; short delay circuit, 8 second; long delay circuit, 2 seconds.
When the selector switch is closed, both storage condensers are charged in the manner described in the above paragraph. (see illustration F5, above). The charge placed on the storage condenser C-1 leaks slowlythrough the resistance R-1 into the firing condenser C-2. The time required for the current to pass to the firing condenser C-2 and build up sufficiently to fire the igniter is called the arming time. At the same time the charge on the storage condenser C-3 is leaking through the resistance R-3 into the firing condenser C-4 and through through the resistance R-2 into the firing condenser C-5. On impact the trembler switches, S-1, S-2, and S-3 momentarily make contact with the trembler cavities causing the current to flow through the igniter bridges. The bridges become heated and fire the match composition surrounding them. When all three igniter bridges fire at the same time the instantaneous bridge fires the flash pellet and detonates the bomb through the normal explosive train. The short and long delay trains just started to burn at the time of detonation.
    If the selector switch is held open, then no charge will pass to the instantaneous circuit but the B plunger, remote from the locating pin, will carry the charge to the storage condenser C-3. The circuit through the resistance R-2 to the condeser C-4 becomes armed before the circuit through R-3 to C-5. If the bomb has been dropped with a falling time of less than 8 seconds, the latter circuit will not be armed before impact and the igniter bridge used with the trembler switch S-2 will fire the long delay pellet which, acting through the explosive train of the fuze, will detonate the bomb. If the bomb is dropped with a falling time of greater than 8 seconds, both circuits will be armed before impact but because of the the shorter delay train used in conjunction with the trembler switch S-4, the short delay will initiate the final explosive train.
    Time fuzes contain essentially the same basic parts as the impact fuzes except that the trembler switches are replaced by a vacuum tube which becomes conducting at a critical known voltage. At the instant the bomb is started on it's trajectory, an electric charge is placed on the storage condenser, and another smaller charge is placed on the firing condenser. The time setting of the fuze is adjusted by varying the amount of charge placed on the firing condenser. During flight, part of the charge on the storage condenser leaks through through the resistor to the firing condenser. As the charge on the firing condenser increases, the voltage across the tube increases. When the firing voltage of the tube is reached, the firing condenser discharges through the tube and igniter, firing the fuze.
    Both the storage and firing condensers of German electrical time fuzes are charged in order that the voltages used can be kept reasonably low. An increased time setting requires an increased voltage.

Advantages of Electrical Fuzes
    A. The fuzes can be set by remote control immediately prior to the instant of firing. Dead time is eliminated in time fuzes, and impact fuzes can be set for instantaneous or delayed actions as each bomb is used.
    B. Electrical impact fuzes are extremely rapid in action because the only mechanical part, the trembler switch, has but little inertia and friction. With an "instantaneous" fuze setting, a bomb can be made to explode without deep penetration of target.
    C. The electrical scheme has great flexibility. The same components were used in a variety of time and impact fuzes having nearly identical appearance but a wide range of characteristics.
    D. The fuzes are safe to handle, ship and store. Whereas in most mechanical fuzes the source of energy to ignite is always present (as a stresses spring, for instance), energy is not introduced into the electrical fuze until the moment of use. Extensive trials were made with voltages as high as 2,000,000 volts, which showed that properly designed electrical fuzes are not affected by atmosphereic electricity.
    E. The electrical fuzes are less expensive to manufacture than mechanical fuzes having the same characteristics, when made in comparable quantities. The cost ratio is about two to three. Another feature which reduces the cost per unit is the fact that parts which are out of tolerance for time fuzes could be used in the impact fuzes.

Methods of Securing the Fuze in the Bomb
    Three methods were used for this purpose; the bayonet joint type, the securing pin type, and the locking ring type. The latter was most commonly used during the war. In this type, the locating ring keeps the fuze in the proper position in the fuze pocket and the locking ring secures it there.

Markings and Identification
    Fuzes generally have the markings on the fuze shoulder, although certain older designs had the markings on the fuze head. The markings on German fuzes must be noted very carefully for in most cases they are the only means of identification..

    The most important marking on the fuze is the number, which is usually circled. This number designates the type of fuze. The units digit is the type number; that is, type 5 fuzes are electrical impact fuzes; type 7 fuzes are time fuzes; type 0 fuzes are protective fuzes, etc. This number is preceded by a number of abbreviations; thus, El. A. Z. means Electrical Impact Fuze. The letter Z is usually found just at the locating pon with the number designation following.
    The "cpp 1941" is the factory designation and the date of manufacture. The "1b" is the factory lot number and the German eagle stamping is the inspector's stamp. Any of the last three may be omitted.

Typical Nomenclature



El. A. Z.
Elektrischer Aufschlagzunder
Electrical Impact Fuze
El. Zt. Z
Elektrischer Zeitzunder
Electrical Time Fuze
El. Z
Elektrischer Zunder
Electrical Fuze
L. Zt.
Long Time (delay)
M. V.
mit Verzogerung
with delay
o. V.
ohne Verzogerung
without delay
vor Beladen entfernen
to be removed before loading
safety time
V. Z.
safety fuzing
fuse extension cap

All information for this entry were acquired from Army Technical manual TM 9-1985-2/Air Force Technical Order TO 39B-1A-9 GERMAN EXPLOSIVE ORDNANCE (Bombs, Fuzes, Rockets, Land Mines, Grenades & Igniters)