Fuseholder

Protection against electric shock (against direct contact with live parts), for fuseholders.

The assessment of the protection against electric shock assumes that the fuseholder is properly assembled, installed and operated as in normal use, e.g. on the front panel of the equipment.

IEC 60127-6 and EN 60127-6 divides into three categories:

a) Closed fuseholder

b) When the fuse carrier is removed, no live parts can be touched.

c) During insertion or removal of a fuse-link no live parts can be touched neither through the fuse-link nor the fuse carrier.

Remarks on PC 3

Influencing factors

The design engineer of electrical equipment is responsible for its safety and functioning to humans, animals and real values. Above all, it is his task to make sure that the state of the art as well as the valid national and international standards and regulations be observed.

In view of the safety of electrical equipment the selection of the most suitable fuseholder is of great importance. Among other parameters, one has to make sure that the maximum admissible power acceptances and temperatures defined by the manufacturer are followed. Differing definitions and requirements in the most important standards for fuse-links and fuseholders are time and again origin for the incorrect selection of fuseholders.

To equate the rated current of a fuse-link with the rated currentof the fuseholder, may, especially at higher currents, causehigh, not admissible temperatures, when the influence of thepower dissipation in the contacts of the fuseholder was not takeninto consideration. 

For a correct selection the follwing influence factors depending on the application and mounting method, have to be taken into consideration.

It is recommended testing the fuseholder with the choosenfuse-link in the worst case operating condition.

1. Rated power dissipation of the suitable fuse-link.
2. Admissible power acceptance, operating current and temperatures of the suitable fuseholder.
3. Differing ambient air temperature outside and inside of the equipment.
4. Electrical load alternation
5. Long time (> 500 h) operation with load > 0.7 In.n
6. Heat dissipation/cooling and ventilation. Heat influence of adjacent components.
7. Length and cross section of the connecting wire.

Rated current of a fuseholder

The value of current assigned by the manufacturer of the fuseholder and to which the rated power acceptance is referred.

Rated power dissipation of the fuse-link

(power dissipation at rated current)

Rated power acceptance and admissible temperatures of a fuseholder.

The rated power acceptance of a fuseholder is determined by a standardised testing procedure according to IEC 60127-6. It is intended to be the power dissipation caused by the inserted dummy fuse-link at the rated current of the fuseholder and at an ambient air temperature of TA1= TA2 = 23 °C (over a long period). During this test the following temperatures must not be exceeded on the surface of the fuseholder:

Illustration of temperatures experienced in practice

TA1 = ambient air temperature, surrounding the equipment
TA2 = ambient air temperature in the equipment
TS1 = temperature of accessible parts on fuseholder surface
TS2 = temperature of inaccessible parts on fuseholder surface

Correlation between operating current I, ambient air temperature TA1 and the power acceptance Ph of the fuseholder.

This correlation is demonstrated by derating curves.

Example of a derating curve

I = operating current of the fuseholder
In = rated current of the fuseholder

The derating curves demonstrate the admissible power acceptance of a fuseholder depending on the ambient air temperature TA1 for the following fuseholder operating currents: I << In, I = 0.7 · In and I = 1.0 · In. This power acceptance corresponds to the max. admissible power dissipation of a fuse-link.

The corresponding values for other operating currents can be interpolated between the existing curves or calculated as follows:

P h = P o - P c = P o - (R c · I 2 )

Ph=  admissible power acceptance in watt of the fuseholder, depending on TA1.
Po=  admissible power acceptance in watt of a fuseholder at I << In, depending on TA1. The values can be taken from the derating curve I << In of the corresponding fuseholder.
Pc=  power dissipation in watt in the fuseholder contacts at the operating current in ampere.
I =  operating current in ampere of the fuseholder.
Rc=  contact resistance in ohm between the fuseholder terminals according to SCHURTER’s catalogue.

Selection of a suitable fuseholder with respect to the power acceptance at the corresponding ambient air temperature.

Summary

The adherence to the limits, indicated by SCHURTER, in particular the power acceptance limits at the corresponding ambient air temperatures and mounting conditions of the fuseholder, is important for the safety of the product. It is therefore necessary to observe the following two steps:

Step 1

Selection of the fuseholder based on the power acceptance.

Ph at operating current I and maximum ambient air temperature TA1.
Pf ≤  Ph = Po - Pc = Po - (Rc · I2).
Pf =  rated power dissipation in watt of the fuse-link, calculated from (In . U), whereas:
In =  rated current in ampere of the fuse-link.
ΔU=  voltage drop in volt at In; values according to SCHURTER's catalog.
Ph, Po, Pc, Rc = see pos. 2.5.

Step 2

The reduction of the power acceptance of the fuseholder (from step 1) based on the different conditions at the mounting place etc. have to be determined by the design engineer responsible.

Examples:

• Ambient air temperature is considerably higher inside of an equipment than outside (TA2 > TA1)

• Cross-section of the conductor, unfavourable heat dissipation

• Heat influence of adjacent components

Therefore, temperature measurements on the appliance under normal and faulty conditions are absolutely necessary.

What's given?

• Fuse-link FSF 0034.1523, rated current In = 5 A. Voltage drop ΔU at In = 80 mV, typ.

Rated power dissipation Pf = (In · Δ) = (5 A · 0.08 V) = 0.4 W.

• Fuseholder FEF 0031.1081, rated current In = 10 A.

 Rated power acceptance at TA1 23 °C = 3,2 W.

• Ambient air temperature = 50 ºC.

 Admissible power acceptance Ph at an ambient air temperature TA1 50 °C according to the derating curve:hA1

Ph at I << In = 2.5W
I = 0.7 · In = 7 A = 2.2W
I = 1.0 · In = 10 A = 2 W

• Contact resistance Rc = 5 mΩc

Solutions

 The result of the interpolation for the rated current I = 5 A is a Ph of approx. 2,4 W.

 The result of the calculation is Ph = Po – (Rc · I2) = 2.5 – (0.005 · 52) = 2.37 W P≈2.4 W.

Step 1

The following condition must be fulfilled:

Pf Ph this means: the rated power dissipation Pf of the fuse-link must be less/equal than the admissible power acceptance Ph of the fuseholder.

Pf = 0.4 W; Ph = 2.4 W at TA1 = 50 °C

Step 2

To consider the different conditions at the mounting place

Conclusion (without consideration of step 2)

• The value Pf is less than Ph. The condition according to formula is fulfilled. It has been chosen a suitable fuseholder.

• If the value Pf were greater than Ph the condition wouldn't be fulfilled. In that case, do select another fuseholder with a higher power acceptance or change the thermal conditions at the fuseholder mounting place.

IEC 60127-6 Fuseholders for miniature fuse-links

NF C93-436 Fuseholders for professional purposes

UL4248-1  Fuseholders

CSA C22.2 NO. 4248.1-07 Fuseholder assemblies

IEC: International Electrotechnical Commission

UL: Underwriters Laboratories Inc. USA

CSA: Canadian Standards Association

NF: French Standard

As mentioned in section 2, the most relevant standards define rated current and rated power acceptance differently. This lead in the past often to confusion or even to a wrong fuseholder design-in.

For example the standard UL 512 does not define a maximum power acceptance value, but sets a certain value of temperature rise for the fuseholder. For this reason the marked amperage values on the fuseholder, defined by UL and CSA, are not suggested to be used except in special cases.

In order to eliminate such confusion, SCHURTER new decided to define the rated current and rated power acceptance values according to IEC 60127-6 and EN 60127-6.

The most important definitions are to be found in section 2.

Conclusion

• The high UL and CSA current ratings are replaced by more realistic rated currents defined by SCHURTER.

• Focused on the new fuseholder standard IEC 60127-6 and EN 60127-6, the power acceptance of several fuseholders had to be reduced.

• The design-in procedure and in particular to choose the correct fuseholder in terms of thermal requirements (refer to section 2-4) is now made much easier.

Your advantages:

• More security for your equipment.

• Faster and much easier selection of the correct fuseholder.

See our Fuseholder products here.