In this fourth and final part, we’ll explain the other parameters for the electrical sizing of Panel boards addressed to systems with the presence of danger of explosion, such as:

1. choice of sectioning and interruption devices;
2. the filiation / coordination;
3. the tripping curves;
4. the differential protection unbalance current (earth leakage);
5. the dissipated power;
6. the preheating.

1. The correct choice of sectioning and interruption devices

This is the first function that the designer must perform properly because it affects the electrical sizing, both the main circuit breaker and the departures.

Notoriously, the sectioning and interruption devices are divided into three classes and more precisely: no load switch – isolator, on load switch – interrupter, thermo-magnetic circuit breakers.

- No load switch – Isolator

These devices are suitable for the sectioning of the power line and can operate in the presence of voltage but without downstream load (defined as sectioning device -time dependent). They cannot open or close in the presence of a short circuit and they do not have a thermal resistance. The No load switch, as an incoming isolating function to the Panel board, it’s unwise, because unable to open with loads (Typical 1 in Scheme 1).

However, it’s possible that the downstream circuit breaker opens without load (Typical 2 in Scheme 1), if the sectioning device is paired with an advanced auxiliary contact on the opening handle. Such contact must be returned to the circuit breaker located upstream of the feeding line, operating on the opening coil of such switch, in order to open it in advance to the downstream isolator, by cutting off the power. In this case, there will be in addition, compared to other solutions described below: a cable, the opening coil and the advanced auxiliary contact.

- On load switch - Interrupter

These devices are suitable for the isolation of the power supply line both in presence of voltage and of nominal load (defined as sectioning device-time independent). They can open and close with the presence of load, but not in the presence of short-circuit. However, they must be able to withstand, without permanent deformation, the short-circuit current passing through.

The on load switch, as an incoming isolating function to the Panel board, it’s recommended as it can open with loads (Typical 1 in Scheme 2). The main reason for this choice is due to the concept of protections coordination. In fact, if there’s a thermo-magnetic circuit breaker upstream, placing an on load switch downstream, ensures the selectivity of action, avoiding the opening under short circuit in one of the two upstream-downstream points, in an unpredictable way (Typical 2 in Scheme 2).

- Thermo-magnetic circuit breakers

These devices are suitable for isolation of the power supply line in the presence of voltage, nominal load, overload and short circuit (defined as sectioning device-time independent). They meet the operating cycle O-CO (according to IEC 60947-2 standard O = open; CO = Closed-Open) and are able to open and close in case of short-circuit.

The thermo-magnetic circuit breaker, as an incoming isolating function to the Panel board, is able to open with the presence of loads and, also, in the presence of short-circuit (Typical 1 in Scheme 3).

Thermo-magnetic circuit breakers can be chosen as general protection of the panel board if it’s not installed another circuit breaker upstream but a device not able to open and interrupt the supply line in presence of over load and short circuit (Typical 2 in Scheme 3). In this case, the upstream main switch (-QG) will protect from over loads and short circuits. This choice is generally not recommended, unless specifically requested by the customer.

If the client does not explicitly specify the type of sectioning device, the designer will select the best equipment considering the above.

2. The correct choice of filiation/coordination

This is another function that the designer must perform properly because it affects the electrical sizing, both the main circuit breaker and the departures.

We assume that the customer gave us the following data:

- One incoming line with thermo-magnetic circuit breaker, four pole (three phase + neutral) (alternatively with ON-load switch, always four pole);

- 10 outgoing feeders with thermo-magnetic circuit breakers, two pole of ~ 9A 230VAC.

We can proceed to select downstream disconnecting devices, with the method of "Filiation".

The method of "Filiation", which until a few decades ago was unthinkable, thanks to the evolution of construction techniques, has made possible to coordinate two or more switching and protection devices in cascade (series), by using the specific characteristics of their limitation power.

The limitation allows to install downstream of an automatic thermo-magnetic circuit breaker, thermo-magnetic circuit breakers with breaking capacity lower than that normally required by a system that does not consider the concept of "Filiation". The upstream circuit breaker performs the function of barrier for high short circuit currents and, limiting such short currents, allows circuit breakers located downstream of being solicited by lower short currents, despite having themselves a capacity of resistance to short circuits lower than the expected short-circuit current requested by the customer at the time of the design development.

This system, as it’s not restrictive between two circuit breakers in series, allows to meet perfectly the needs of the customer (filiation corresponding to the requirements imposed by CEI EN 60947-2 and CEI 64-8 standards).

The protection of such equipment shall be coordinated so that the specific pass-through energy (I²t) from the upstream protective device should not be higher respect to the energy which can be withstood without damage by the downstream protection device and the electrical connections.

This system can be validated only if are used switches of the same brand, since that function must be verified by laboratory tests that can be carried out only by the manufacturer. Therefore, the possible combinations can be checked in the manufacturer's technical documentation.

We can now analyze the sizing using, as per the first hypothesis, a thermo-magnetic circuit-breaker as a main circuit breaker.For this sizing, we’ll evaluate the products manufactured by Schneider Electric, Acti9 series, model iC60 (in this analysis will not be taken into account the C40N series switches because not suitable to be operated from the outside of the explosion-proof enclosures).The correct choice of the switching devices is one of the various functions that the designer must perform properly, as it influences in a critical way the electrical system.

The first usable size is the circuit breaker iC60N, with a Icu of 20kA (redundant to the value required by the customer) and, by selecting from the table the downstream coordinated circuit breaker, from 9A to 45°C, downgraded to such use, we will adopt a circuit breaker with a rated current of 16A at 30°C. It will be suitable for 9A at 45°C, or rather iC60a 16A, that with the concept of "Filiation", shall ensure the coordination with reinforced Icu 20kA, despite having one of Icu 10kA, as shown in Scheme 4.

Let's now look at the sizing considering the second hypothesis (alternative requested by the client), using as incoming main switch, a on-load isolator switch with the same data examined for the version with the circuit breaker described above.

Premise: in this case, as we haven’t an incoming thermo-magnetic circuit breaker for the protection from overloads and short circuits, the client have to inform about the type of circuit breaker located upstream of the supply line, the distance (in meters) and the section and the nature of the power cable, in order to perform the correct sizing of the circuit breakers (departures) located downstream respect to the incoming on-load switch.

For this sizing, we’ll examine the products manufactured by Schneider Electric, Acti9 series,  iSW -NA model with free-release.

The on-load isolator switch is able to interrupt only the rated current and it’s able to withstand the stresses resulting from the electromagnetic short-circuit current. Therefore, it’s not able to open in the presence of an overload or a short circuit. For this alternative, the ability of opening in the presence of an overload or short circuit will be delegated to the circuit breaker located upstream of the Panel board supply line.

The first parameter to be checked is the electrical size of the on-load isolator switch which must have the value of rated current equal to or greater than the rated current of the circuit thermo-magnetic circuit breaker upstream of the power line.

Another parameter to be checked is that the on-load isolator switch is appropriate for a thermal resistance to short circuit (Inc) greater than or equal to the short circuit current required by the customer and/or to the short-circuit current of  upstream thermo-magnetic circuit breaker.

Assuming that you have a thermo-magnetic circuit breaker upstream of 32A at 400VAC at 45°C, with a interruption capacity of 10kA Icu, according to the below Table 2, which for convenience we quote in full, for such coordination we have to select a on-load isolator switch with free-release of 63A, which downgraded for temperature, it’s able to withstand a current of 32A at 45°C (for convenience we will adopt the same engineering parameters set out in Part Three of the mechanical and electric design). So will be adopted an on-load isolator switch with a nominal current of 63A at 30°C.

In regards to the above, that is to have one on-load disconnecting switch incoming to the Panel board, we need to assess what value of short circuit current must be taken into account to properly size the circuit breakers downstream of the on-load isolator switch.

Assume that we have received from the customer the size of the power cable, the distance and the voltage drop, as shown in Scheme 5 above. As reported in Table 3 below, which we quote in full, using a cable with a section of 25mmq and distance of 160m (154,2m by defect), you will have a short-circuit current on the busbar of the Panel board of 2kA; therefore, the circuit breakers -QF1 ÷ -QF10 will be sized for this short-circuit current.

What described above is valid in the case that the customer has not provided the value of incoming short circuit current but it has communicate the characteristics of the upstream circuit breaker, the cable section and the distance from the upstream Panel board.

In the event that the customer is aware of the value of incoming short-circuit current, what described above have to be redefined as a function of this parameter and sizing the on-load isolator switch in order to withstand, without permanent deformation, the short-circuit requested.

3.The correct choice of the tripping curve

As already briefly described in the first part, the choice of the tripping curves of the automatic thermo-magnetic or magnetic only circuit breaker is also an important part in the sizing in order to protect the underlying loads. It’s a parameter, as for the power interruption (Icn), which must be communicate from the customer at the time of sizing request.

The main function of a thermo-magnetic circuit breaker is to ensure the protection of circuits against overloads and short circuits.

- Overloading: this function is obtained by thermal bimetallic releases or by the static releases at inverse time associated with the circuit breaker, depending on the size of the circuit breaker chosen.

- Short circuit: this function is obtained from magnetic or static releases time-independent, instantaneous or with a short delay, depending on the size of the circuit breaker chosen.

From the association of bimetallic thermal releases with magnetic releases in the same circuit breaker, result commonly thermo-magnetic releases. If the circuit breakers should play only the function of short-circuit protection, are commonly called magnetic circuit breaker (specific for motor protection, with typical curve MA).

The modular circuit breakers, in general, are equipped with thermo-magnetic releases integrated in the structure of the circuit breaker and, therefore, not interchangeable. They have no possibility of adjustment of the currents of intervention by users, but, being available in a wide range of calibrations, they are able to satisfy all the sizes of the downstream loads.

Releases are available with different types of intervention curves in relation to different possible uses such as:

• Curve C: cables and plants feeding classic appliances
• Curve D: when the starting current is of considerable intensity (5 to 7 times the rated current)
• Curve K: protection of uses with high starting currents (motors, transformers)
• Curve Z: protection of electronic circuits
• Curve MA: motor protections (circuit breakers with magnetic function only).

The rules that govern the design, performance and testing of circuit breakers for overcurrent protection are:

- The standard IEC 60947-2, which is the reference text for the products for "industrial" applications, with high values of interruption capacity and features that meet the needs of safety and proper operation of electric plants in the manufacturing sector.- The standard CEI EN 60898-1 which applies to circuit breakers for household and similar uses, including applications for offices, schools, hotels, etc.. etc.., commonly referred as "tertiary sector".

In our case, we’ll be taken into consideration only the circuit breakers conforming to the standard IEC 60947-2, as the products are addressed to manufacturing plants, oil extraction, chemical or chemical-pharmaceutical plants and, in general, all those plants that present a risk of explosion due to the presence of gas or explosive dusts.

4. The choice of the correct differential protection imbalance current

This is an important parameter for the protection of persons and things.Considering that the risks arising from the physical contact with the electric current has a negative effect on the respiration and circulation, also the risk of burns produced by the passage of current through the body should not be underestimated. Whatever the neutral system is, in the case of a direct contact, the current that returns to the source of energy is the one that passes through the human body.The limit of perception varies from one subject to another, from a minimum less than 1mA to a maximum of 2mA, while muscle contraction, for frequencies from 50 to 100Hz, varies from 10mA for women, up to 15mA for men, whereas these values may vary depending on age, sex, state of health etc…

In practice the respiratory arrest occurs with currents from 20 to 30mA, causing respiratory apparatus muscle contractions up to cause the arrest of breathing.

The cardiac fibrillation varies from 70 to 100mA, depending on the different body characteristics such as the weight, but in reality, it’s not definable in a precise way because of the physiological and environmental conditions of the subject such as the current path through the body, the type of contact, the contact time and the value of resistance of the organism.

As mentioned above, other important risk are the burns resulting from contact with electricity generated by the electric arc that causes excessive and dangerous heat to the human body.

The principle of the differential protection of Schneider Electric is based on a system capable of ensuring, almost instantly, three functions in sequence, such as:

- detection of leakage current, obtained by a toroidal current transformer (Scheme 6) in which the primary is represented by the conductors of the circuit to be protected. Under normal conditions, the vector sum of the currents passing through the conductors is zero, therefore the cash flows generated inside the toroid annul each other. The occurrence of a leakage current breaks this balance and induces a residual current in the secondary.

- Measure of the same, made by an electromagnetic relay (Scheme 6) that compares the electrical signal received from the toroidal current transformer with pre-intervention levels (sensibility). The principle of operation of the relay is the following: an electromagnet powered by the residual current transmitted by the toroid, exerts a force on the release mechanism which is opposed to that exerted by a permanent magnet to hold the contacts in the closed position. Until that the force of the permanent magnet is superior to the electromagnet one, the circuit remains closed.- Interruption of the circuit affected by a failure, which occurs when the residual current is high enough to cancel the effect of the permanent magnet and, then, the release mechanism controls the opening of the contacts, thus breaking the circuit in which the failure occurred. The differential devices Acti 9 series Schneider Electric are of the electromechanical type with current operation, because most secure: it’s independent from the mains voltage and, especially, it does not require any external voltage source.

5. The proper sizing of the enclosure in function of the power dissipation

This is another parameter that the designer should consider, as this parameter is one of the fundamental pillars in order to respect the class of temperature where it will be installed.

The dissipation power of the component and/or equipment, at the ambient temperature of the project (W), is the value of thermal dissipation of such object which is usually stated by the manufacturer of the object. This value must be evaluated again, in function of the operation at a specific design temperature, whereas normally the components and / or equipment have a value of dissipation as a pose in free air and then at a temperature of 20° C or at other temperature stated by the manufacturer.

"Temperature class" means the value, expressed in °C, to which to refer for the control of the maximum surface temperature that the outer surface of the housing can reach for its correct operation in an environment classified as:

• T1 = 450°C
• T2 = 300°C
• T3 = 200°C
• T4 = 135°C
• T5 = 100°C
• T6 = 85°C

Moreover, this sizing must be in full compliance with the maximum allowable dissipation value in accordance with the Ex certificate of the specific enclosure at the temperature class and the relative environmental temperature.

To proceed with the sizing, the following parameters must be considered:

• electrical and/or electronic equipment devices shall conform to IEC or EN rules;
• the values of power dissipation for each equipment shall also consider the contribution of dissipation of electrical connections;
• have a minimum distance between no ‘Ex i’ and ‘Ex i’ internal wiring, satisfying the requirements of the technical note specific of each type of enclosure;
• have an average thickness of the insulators of the conductors superior or equal to 0.7mm;
• have a minimum cross-section of the conductors not less than 1,5mmq;
• have a minimum distance between the walls and the equipment in compliance to the specific technical note of each type of enclosure;
• have an open area in each cross-section, not less than 30% but still in full compliance with the requirements in the specific technical note of each type of enclosure;
• have a minimum distance in each direction, of uncovered conductive parts of ‘Ex i’ and non ‘Ex i’ connectors, not less than 50mm (surface distance);
• have the conductors insulation of intrinsically safe circuits not less than 500V;
• have enough space between equipment, such as to provide a wiring  "according to the rules of art”;
• have defined the coefficient of contemporaneity between the various equipment.

After observing the above requirements, taking the values of dissipation of each equipment, you can then proceed to calculate the total dissipation by adopting the simple formula: R1 + R2 + ... ..Rn = Rt where R1 ... Rn are the values of dissipation of the individual components and Rt is the total value of dissipation for parallel operation of the equipment.

Of course, all the values of dissipation must be taken from the technical documentation of the manufacturer and measured at the project ambient temperature.

Obtained the value of total dissipation "Rt", you have to compare it with the maximum allowable dissipation of the selected enclosure above (see Part two) regarding the maximum capacity of inlets. If this enclosure will fit both the number of entrances and the maximum allowable dissipation, the sizing will be concluded.

6. Sizing the preheating

This is a variable function that is applied only when the ambient temperature is negative (from -20°C up to -60° C). The designers have to consider it, even if not expressly requested by the customer, but necessary for the correct operation of the equipment at this negative temperature.

All electrical and electronic equipment are built to operate at specific climatic conditions which may vary from manufacturer to manufacturer but, in practice, the range is from -20° C to + 40° C.

When the temperature is positive, must be carried out, as previously described, the realignment at the design temperature, compared to the functional temperature stated by the manufacturers.

When the temperature is negative, must be carry out the verification with the values declared by the manufacturers and, if this temperature does not fall within the design value, a pre-heating must be provided in order to ensure the full functionality of the equipment.

For illustrative purposes, we make a practical example.

We can suppose that must be installed a Panel board with an ambient temperature of -60° C using Acti9 series circuit breakers of Schneider Electric, with a minimum operating temperature of -35° C.

In this case, the circuit breakers have to operate at a temperature considerably lower than the temperature allowable by the equipment. It will be necessary to provide for a pre-heating system that brings back the temperature at the equipment minimum allowable temperature.

To do this, it’s need a system of resistors that raise the internal temperature from -60° C to the functional temperature of -35° C, in order to ensure proper operability of the interior equipment.

For the purposes of calculation, the following parameters should be considered:

internal dimensions of the enclosure;

• thickness of the metal;
• material of the enclosure;
• coefficient of thermal conductivity of the enclosure material;
• inside air convection (if applicable);
• outside air convection;
• internal/external surface;
• internal request temperature;
• external project temperature;
• material from external insulation (if required).

Based on these data, using specific calculation, will be determined the delta T° C, the overall heat transfer coefficient, the power required in Kcal and power required in Watts.

Obviously, as the delta temperature is from -60° C to -35°C equal to 25°C, the pre-heating system must be sized for this value of delta T, so as to ensure the operation of the equipment without permanent damage.

As you can note in the Scheme 7 below, in order to ensure the functioning of the equipment at a ambient temperature of -60° C, it will be necessary to insert, downstream of the main circuit breaker, a contactor which, controlled by the thermostat differential -B, will control the opening and closing of the contactor -K and, consequently, the switching on and off of the pre-heating resistor -R. Through the circuit breaker -QK, will be possible, in the summer period, to exclude the pre-heating resistor, simply operating on the opening of the same that, through an auxiliary discordant contact, powers the -K contactor in allowing the operation of the Panel board.

Remark: The diagram is shown with open circuit breakers and in the absence of voltage

Obviously, as it’s an explosion-proof Panel board, it can never be opened if not after removing the voltage to the supply line, acting on the upstream switch.

Hoping to have given a useful descriptive tool in the four sessions, with the all variables involving the designer during the stage design, we conclude this fourth and final drafting saying that all the electro-mechanical sizing activities are the prerogative of the manufacturer of the Panel Boards in explosion-proof execution: analysis, calculations and the resulting executive project are of its responsibility, putting the plate certifying the compliance with the relevant Standards.