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:
Fuseholders without integral protection against electric shock.
They are only suitable for applications
where corresponding additional means are provided to
protect against electric shock.
Fuseholders with integral protection against
live part is not accessible:
- when the fuseholder is closed
- after the fuse carrier (incl. fuse-link) has been removed
- either during insertion or removal of the fuse carrier (incl. fuse-link)
Compliance is checked by using the standard test finger
specified in IEC 60529.
Fuseholder with enhanced integral protection
against electric shock
The requirements for this category are the same as those
for category PC2, with the exception that the testing is
carried out with a rigid test wire of 1 mm diameter
accor ding to IEC 60529, table VI, instead of the standard
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
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:
Fuseholder surface area
Maximum allowable temperature
1. Accessible parts 1)
(see figure 1) °C
2. Inaccessible parts 1)
Illustration of temperatures experienced in practice
TA1 = ambient air temperature, surrounding the equipmentA1
TA2 = ambient air temperature in the equipmentA2
TS1 = temperature of accessible parts on fuseholder surfaceS1
TS2 = temperature of inaccessible parts on fuseholder surfaceS2
Correlation between operating current I, ambient air temperature TA1 and the power acceptance Ph of the fuseholder.A1h
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.hA1
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.onA1n
Pc= power dissipation in watt in the fuseholder contacts at the operating current in ampere.c
I = operating current in ampere of the fuseholder.
Rc= contact resistance in ohm between the fuseholder terminals according to SCHURTER’s catalogue.c
Selection of a suitable fuseholder with respect to the power acceptance at the corresponding ambient air temperature.
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:
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)fhococ2
Pf = rated power dissipation in watt of the fuse-link, calculated from (In . U), whereas:fn
In = rated current in ampere of the fuse-linkn
ΔU= voltage drop in volt at In; values according to SCHURTER's catalog.n
Ph, Po, Pc, Rc = see pos. 2.5hocc
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.
• ambient air temperature is considerably higher inside of an equipment than outside (TA2 > TA1)A2A1
• 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.
• Fuse-link FSF 0034.1523, rated current In = 5 A. Voltage drop ΔU at In = 80 mV, typ. nn
Rated power dissipation Pf = (In · Δ) = (5 A · 0.08 V) = 0.4 W.f
• Fuseholder FEF 0031.1081, rated current In = 10 An
Rated power acceptance at TA1 23 °C = 3,2 W.A1
• 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
The result of the interpolation for the rated current I = 5 A
is a Ph of approx. 2,4 W.h
The result of the calculation is
Ph = Po – (Rc · I2) = 2.5 – (0.005 · 52) = 2.37 W P≈2.4 W.hoc
The following condition must be fulfilled:
Pf Ph this means: the rated power dissipation Pf of the fuse-linkfh
must be less/equal than the admissible power acceptance
Ph of the fuseholder.
Pf = 0.4 W; Ph = 2.4 W at TA1 = 50 °CfhA1
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.fh
• 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.fh
IEC 60127-6 Fuseholders for miniature fuse-links
NF C93-436 Fuseholders for professional purposes
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.
• 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.
• More security for your equipment
• Faster and much easier selection of the correct fuseholder