5.1 Working Voltage
An important parameter when selecting a suitable isolator or transmitter is the allowable working voltage. It indicates the allowable potential difference between the different circuits (input, output and supply circuits) (see Fig. 21).
Figure 21: Illustration of the significance of the working voltage
The permitted working voltage of an isolator may differ between the different circuits. The highest of the specified working voltages generally applies across the input and output and across the input and power supply. The permitted working voltage across the output and power supply can be lower. The allowable working voltages for Knick isolators are up to approx. 3600 V AC/DC depending on the model.
The requirements for the working voltages are defined in EN 61010 Part 1. It should be noted that the allowable working voltage is not only determined by the dielectric strength of the solid insulation used for the isolating components, but also by design features such as clearance and creepage distances as well as by external influences, particularly the pollution degree (environment-related, possible contamination) and the overvoltage category (value of superimposed pulse voltages).
Pollution Degree 1
There is no or only dry, non-conductive contamination, for example in air-conditioned or clean, dry rooms.
Pollution Degree 2
There is only non-conductive contamination. Occasional, temporary conductivity due to condensation can be expected, for example in laboratories, precision mechanics workshops, test departments and sales rooms.
Pollution Degree 3
There is conductive contamination or dry, non-conductive contamination that becomes conductive due to expected condensation, for example in rooms at industrial plants, agricultural plants, unheated warehouse rooms and boiler rooms.
Pollution Degree 4
The contamination leads to permanent conductivity, for example, due to conductive dust, rain, or snow; in open rooms or outdoors.
Overvoltage Category I
No occurrence of overvoltage or protection by surge arresters or filters.
Overvoltage Category II
Systems with switching processes, but without lightning strikes, for example, in private households.
Overvoltage Category III
Systems without lightning strikes, connection of the device nearer to the power supply connection than the loads and/or special requirements for safety and availability of the device.
Overvoltage Category IV
Plants with lightning strikes.
5.2 Test Voltage
The dielectric strength of the insulation material used for the isolating components is many times greater than the working voltage permitted for the device. Therefore the test voltage with which each device is tested is specified additionally for potential isolating devices. This ensures that the specified limit values for the working voltage apply to each device. The test voltage is also used occasionally as a parameter for the dielectric strength instead of the working voltage. This then needs to be a certain factor higher than the maximum possible potential difference between the circuits to be isolated according to the directive for the specific application. The test voltage for Knick isolators is up to 15 kV AC.
5.3 Protective Separation
The term “protective separation“ is defined in EN 61140. This basic safety standard describes protective measures against electric shocks and thus defines the requirements to be met by electrical isolation between electric circuits. "Protective separation" refers to an isolation system which is characterized by basic protection and additional protection (additional insulation or protective shielding) or by equivalent insulation (reinforced insulation according to EN 61010-1). It has the aim to protect persons against dangerous body currents. This particularly high level of safety must be ensured by constructive measures (such as appropriate clearance and creepage distances) as well as by the insulation properties of the internal isolation components. That means that the requirements of the basic safety standard have a direct effect on the isolator design.
The specification “protective separation“ always includes the indication of a working voltage up to which the “protective separation“ is guaranteed. All new isolators and transmitters developed by Knick meet the requirements of EN 61140 regarding “protective separation“.
On an international basis, often the US and Canadian UL approvals (Underwriter Laboratories) and CSA (Canadian Standards Association) or their combination CUL are demanded. Many Knick isolators have proven in strict approval tests that they fulfill the respective requirements for electrical safety and fire protection.
The GL approval (Germanischer Lloyd) confirms the high load capability of many Knick isolators. It permits the use on ships and for offshore applications, for example. The “Kerntechnische Ausschuss“ (KTA - Nuclear Safety Standards Commission) in the German Federal Office for Radiation Protection issues nuclear safety standards that are identified by the letters KTA. Knick offers isolators with KTA 3503 (3507) approval for application in nuclear power stations.
5.5 Hazardous-Area Separation
When electrical equipment (including electrical and electronic measuring devices) is used in hazardous areas, it is necessary to ensure that these devices cannot ignite any explosive gas-air mixtures even when they malfunction. The possible types of protection include the “intrinsic safety“ that ensures that the intrinsically safe circuits in the respective device cannot generate ignitable sparks even in the event of a malfunction.
In practice, an isolated solution with intrinsically safe circuits is hardly ever possible, i. e. coupling with non-intrinsically safe circuits is necessary for signal processing. The direct link would cancel out the intrinsic safety and is therefore not permitted. Knick supplies suitable externally or loop-powered isolators for coupling the corresponding circuits, which provide both electrical isolation and separation between intrinsically safe and non-intrinsically safe circuits (see Fig. 22).
Figure 22: Coupling of intrinsically safe and non-intrinsically safe circuits with the IsoTrans 36 A7 passive isolator
The devices supplied by Knick with “intrinsic safety“ type of protection comply with the 94/9/EC directive. This directive is generally referred to as ATEX.
In practice, category 1 and 2 devices are used for hazardous areas within Europe, so devices for zone 0 and 1. Equipment of Category 3 – for application in Zone 2 – is becoming increasingly important. Zone 2 is a place in which an explosive atmosphere consisting of a mixture of air with hazardous substances in the form of gas, vapor, or mist is not likely to occur in the normal operation and, if it does occur, will persist for a short period only. Normal operation means the situation when systems and devices are used within their design parameters as specified by their manufacturer. Such systems and devices provide the required safety level.
Zone 2 may include, among others:
– Areas surrounding Zones 0 or 1,
– Areas around flange connections with ordinary flat gaskets in pipes in enclosed spaces.
The Series P32x00 and A202x0 transmitters are intended for operation in ATEX Zone 2 (EN 60079-15) or
Class 1, Div 2 / Zone 2 (UL 1604).