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Posted on 21 April 2019

Parallel and Series Connection of Fuses

 

 

 

 

 

 

 

Fuses connected in parallel

In North America, the Electric Codes do not permit the paralleling of overcurrent protection devices in the field, although paralleling is permissible in a factory built assembly.

Current rating:

When two fuses are connected in parallel it is necessary to ensure that they share the current equally, by making the connections to the terminals symmetrical and of equal impedance. A derating factor of 0.9 may be necessary, depending upon the fuse type and the installation.

Voltage rating:

In some cases the voltage rating must also be reduced by 10-15% for certain fuse designs.

Mechanical problems:

Another potential problem  is the possibility of mechanical damage due to variations in the length of the fuse. Variations of the length of the fuses are mainly due to variations in the length of the body. When trying to make  contact with all of the fuses, the contact of one fuse is pulled and the fuse elements inside the fuse are badly stressed. Some elements can break.  In an  example with three fuses in parallel, this problem is obvious, but the same can occur with two fuses in parallel. Therefore, when two fuses are connected in parallel, the connection on one side of the fuses must be flexible in order to avoid mechanical stress leading to damages to fuse elements inside one of the fuses.

CONCLUSION:

Before connecting two or more fuses in parallel, it is necessary to consult the fuse manufacturer to determine the appropriate voltage and current ratings. It is also necessary to determine  the acceptable difference in resistance values of the fuses for parallel connections. Potential mechanical problems as decribed above should also be avoided.

Fuses Connected in Series

A gerneral rule to follow when connectingfuses in series is that the voltage rating of the fuse must be equal to or greater than the RMS (root mean square) value of voltage of the faults cleared by the fuse.The main reason for this statement is that fuses will melt simultaneously when the fault current is large enough.

When the fault current is not large enough one fuse will melt several milliseconds before the other one so that the arcing energy is mostly or totally in one fuse. This can result in very bad consequences such as explosion of the fuse or fuse venting.

When two fuses are connected in series for fault interruption, the calculation of the I²t for each fuse takes into account the benefit of there being two fuses, and the voltage across the fuse is taken to be half of the fault voltage. However, inside some converters the fuses do melt exactly at the same time and a 30% imbalance factor is introduced. As a result, the I²t correction factor is calculated for 65% the fault voltage.

 

For more information, please read:

Introduction to Semiconductor Fuses

Semiconductor Fuses – General

Semiconductor Fuses: Terms and Explanations

Fuse Placement in Typical Converter Circuits

Dimensioning Semiconductor Fuses

 

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One Response

  1. avatar Dave Dean says:

       Most of us are familiar with resistors in series or parallel and capacitors in the same

    situation. Certain laws apply.   I recently had occasion  to install two ten amp liilefuses

    in series as part of two 12V plug - ins for mobile usage.....piggy-backed to the old familiar

    `cigarette lighter` ( 12AV outlet in a car ).  The first connected plug is a dual unit with a

    10A fuse allowing two plug - ins to itself. The next plug is a single plug, leaving another

    opening for a second plug - in. My question is with two 10A fuses in series ( or with a

    second plug - in  there would be wye with each load in series with the dual unit ).  I simply

    wonder what the consequence would be.....with all fuses identical, what kind of

    protection I would have.  My theory is that at least one of the fuses would blow in

    the case of an overload or short and there is no significant drawback,  but I am not

    afraid to ask another opinion!   73, Dave K1JGV

     

     

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