Two of the most fundamental ratings assigned to an electric cable are Voltage Rating and Temperature Rating.
Voltage Rating
The rated voltage of a cable is the reference voltage for which the cable is designed, and it also helps to define which electrical tests are carried out.
Earthed Neutral Systems
This is the most common type of system encountered.
The rated voltage for power cables is normally expressed by the combination of two values Uo/U, expressed in volts:
- Uo being the r.m.s value between any insulated conductor and ‘earth’ (metal covering of the cable or the surrounding medium);
- U being the r.m.s value between any two phase conductors of a multicore cable or of a system of single core cables.
In an alternating current (a.c.) system, the rated voltage of a cable shall be at least equal to the nominal voltage of the system for which it is intended. This condition applies to both the value of Uo and to the value U.
In a direct current (d.c.) system, the nominal voltage of the system should be not higher than 1.5 times the rated voltage of the cable.
Where it is allowed by the applicable regulation or legislation some types of cables may be used where:
On cables up to and including 450/750 V the operating voltages of a system may permanently exceed the nominal voltage of such a system by 10 %.
On cables rated at 600/1000 V and higher, the operating voltages of a system may permanently exceed the nominal voltage of such a system by 20 %.
This value is known as Um
Popularly recognised voltage ratings are:
Low Voltage (LV) power cables
- 300 / 500 V (or 0.3/0.5 kV)
- 450 / 750 V (or 0.45/0.75 kV)
- 600 / 1000 V (or 0.6/1.0 kV)
But much higher voltage ratings are used for large, high-power equipment and machinery, examples of these IEC ratings being:
Medium Voltage (MV) power cables
- 1800 / 3000 V (or 1.8/3.0 kV)
- 3000 / 6000 V (or 3/6 kV)
- 6000 / 10000 V (or 6/10 kV)
- 8700 / 15000 V (or 8.7/15 kV)
- 12000 / 20000 V (or 12/20 kV)
- 18000 / 30000 V (or 18/30 kV)
High Voltage (HV) power cables
- 36000/66000 V (or 36/46 kV)
- 64000/110000 V (or 64/110 kV)
- 76000/132000 V (or 76/132 kV)
- 127000/220000V (or 127/220 kV)
- 190000/345000V (or 190/345 kV)
- 220000/400000V (or 220/400 kV)
Un-earthed Neutral Systems
There are certain more specialised applications where for various reasons an un-earthed neutral system is used. In these systems, the voltage rating of the cable should be U/U, not the normal Uo/U. Because special cables with a voltage rating U/U are rarely, if ever made, it is usual to select a cable with the next highest earthed neutral rating, for use on an un-earthed neutral system. For example, for a 10 kV unearthed neutral system it is usual to select an 8.7/15 kV rated cable, although importantly this should always be checked against the circuit protection system.
Other non-power cable types
The rated voltage for non-power, more specialised types of cables such as Control and Instrumentation cables, Data cables and telecommunication cables are commonly identified by just one nominal voltage rating, typically 60 V, 300 V, or even 500 V.
IMPORTANT NOTE: These cables are not intended for direct connection to mains electricity supply or other low impedance sources. These types of cables are only intended for use in analogue, digital and control circuits and are not to be used for power supply.
Conductor Temperature Rating
All Thermoplastic and Thermoset materials used to make cables will slowly age and deteriorate over time; the rate of deterioration will accelerate as the temperature increases (Arrhenius Law). It is therefore usual to state a temperature at which the material will continue to operate normally and give an acceptable life; for electric cables, this is known as the “continuous conductor operating temperature” rating.
The continuous conductor operating temperature for the popular insulation materials used by Tratos are:
- Ordinary PVC 70 C
- High-temperature PVC 85/90 C*
- Cross-linked Polyethylene 90 C
- EPR/HEPR 90 C
- Thermoplastic Zero halogen low smoke 70 C
- Thermoset Zero halogen low smoke 90 C
* Some PVC compounds are rated at 105oC but this is usually a short life rating used in some American Standards.
These “continuous conductor operating temperature” ratings are based on the proven experience and reliability of cables over many years of operation.
The “continuous conductor operating temperature” rating is important because as well as being directly related to the acceptable life of a cable, it is directly related to the Current Rating of a cable.
Current Ratings
Someone once said, “Electric cables are inefficient electric fires” and when one looks closely at the subject they are not all that wrong!
All conductors have a measurable value of resistance. When a cable is energised and a potential difference (voltage difference) exists between points in a circuit then a current will flow. The mere act of this flow of current having to overcome the resistance of the conductor is a form of work, this work generates heat in the conductor and this heat must go somewhere.
The insulations, coverings and finishes that we apply to conductors act as heat barriers – thermal insulation – and to a greater or lesser degree prevents this heat from freely escaping, radiating, from the conductor. The installation also has an important bearing on the current rating of a cable, any installation where the heat generated cannot freely radiate away from the cable, such as cable in conduit or cables under thermal insulation in attics, must be derated accordingly.
Different sizes of conductors have different resistances and the various materials we use to cover them have different maximum continuous conductor temperature ratings.
Therefore there is a direct relationship between:
- Conductor size
- Insulation and coverings
- The current Rating of a particular cable
An important point to note is that the maximum continuous conductor temperature is derived from initially, the ambient air temperature surrounding the cable, to which must be added the temperature rise caused by the flow of current.
It must also be remembered that although the foregoing is true for all types of cable, in practice it is only applicable to power cables or cables carrying appreciable amounts of current, 10’s or even 100’s of amps. Data, Control and Instrumentation cables, which usually only carry only fractions of an amp, are not normally sized on the current that they will actually carry, but on other factors such as voltage drop and mechanical considerations.
Power over the ethernet (POE) is now causing people to look more closely at what currents data cables can carry safely without deterioration in transmission characteristics.
Tratos catalogues and web pages contain useful information regarding the temperature rating and current rating of the various cables that we produce, Tratos Engineers are also always available if specific advice is required.
