Capacitive Barrier Evaluation Application Note The Solexy AX series

Hazardous Location Telemetry Equipment TM
Capacitive Barrier Evaluation
Application Note
10178 International Blvd
Cincinnati, Ohio 45246
Phone: 513-860-5465
Fax: 513-860-5464
www.solexy.net
Page 1 of 6
The Solexy AX series antenna coupler is a component device and must be evaluated in the installed equipment.
This device is certified as Ex d mb [ia] (mining and gas applications) and Ex mb [ia] (dust applications),
meaning that the coupler itself is Ex d mb or Ex mb and that the internal electronics provide Ex ia outputs to
the antenna connector. The following conditions must be considered during installation:
1) Combined radio and antenna effective radiated output power cannot exceed Group I, IIA, III
6W
Group IIB
3.5 W
Group IIC
2W
2) Radio output power cannot exceed 1W 3) Um Supply Voltage cannot exceed 250 VDC or 250 VAC 50/60Hz The following formulas can be used when evaluating the coupler:
Formula 1: Output voltage from the radio (Um(radio)) is based on the radio power driven into a 50Ω
impedance load as calculated per the below formula.
√ ∗ 50Ω
Formula 2: The reactance of the coupler (Xc) can be determined based on the following formula:
1
2π ∗ C ∗ f
Formula 3: The frequency (f) of the coupler can be determined based on the following formula:
1
2 ∗
∗
Formula 4: The output current (I) from the coupler can be determined based on the following formula:
Formula 5: The output energy (E) from the coupler can be calculated based on the following formula:
1
∗
2
∗
AN00001-01
Hazardous Location Telemetry Equipment TM
Capacitive Barrier Evaluation
Application Note
10178 International Blvd
Cincinnati, Ohio 45246
Phone: 513-860-5465
Fax: 513-860-5464
www.solexy.net
Page 2 of 6
where, P = Radio Power
C = Capacitance (see Note 1 below)
f = Frequency
Um = Maximum Supply Voltage (see Note 2 below)
XC = Coupler Reactance
Notes:
1) The maximum capacitances to be considered in an assessment of a system is as follows:
Table 1
Model Capacitance AXF3S****AX
AXFMS****AX
AXN3S****AX
5.65nF AXNMS****AX AXF3S****JX
AXFMS****JX
AXN3S****JX
81.6 pF AXNMS****JX AXZ3S0400ZX 2.16 pF 2) When evaluating the coupler, it is necessary to assess all other voltage/ frequency sources supplied by the input
circuits individually. This could be a source from crystal oscillators, microprocessors, switch-mode power
supplies, etc., as well as the mains supply voltage. See Table 2 for details on evaluating the maximum voltage
vs. various frequencies.
3) When AC signals combine they are still considered as individual entities and are not additive as they are with
DC signals. Therefore, any of the combined voltage frequencies that enter the I.S. barrier should be evaluated
independently using Table 2.
4) Voltages or frequencies outside of those specified in Table 2 are not permitted.
5) On an FM radio, the output current and voltage are derived from the power amp, which is limited to a specific
power and must be under 1 Watt. The voltage output of the radio, as calculated according to Formula 1, will
not exceed the operational voltage of the radio as the radio output is tuned to a specific load such as 50 Ohms.
AN00001-01
Capacitive Barrier Evaluation
Application Note
10178 International Blvd
Cincinnati, Ohio 45246
Phone: 513-860-5465
Fax: 513-860-5464
www.solexy.net
Hazardous Location Telemetry Equipment TM
Page 3 of 6
Example 1: Determine short-circuit current based on supply voltage
When the maximum supply voltage of 250VAC is applied to the input of the coupler using the 5.65nF
capacitance, the short-circuit current is determined by using the supply voltage frequency of 50Hz driving a
5.65nF capacitive load. The resulting circuit is shown in Figure 1.
C
5.65nF
Um 250V @ 50Hz
I
Figure 1: Short-Circuit Current at Max Um
The short-circuit current can be found as follows:
1
2πCf
1
2π ∗ 5.65nF ∗ 50Hz
250
563.380 Ω
563.380 Ω
448μ
This current is well below the curve for IIC applications at 250V and is considered satisfactory.
AN00001-01
Hazardous Location Telemetry Equipment TM
Capacitive Barrier Evaluation
Application Note
10178 International Blvd
Cincinnati, Ohio 45246
Phone: 513-860-5465
Fax: 513-860-5464
www.solexy.net
Page 4 of 6
Example 2: Determine maximum allowable coupler frequency based on supply voltage
With a maximum supply voltage of 133V, Figure A.1 of IEC 60079-11 shows that 20mA is allowed. Applying
a 1.5 safety factor on this current allows 13.3mA. Analyzing with a maximum capacitance of 5.65nF:
133
9975Ω
13.3
1
1
2.82
2π ∗ C ∗ Xc 2π ∗ 5.65nF ∗ 9975Ω
This result shows that, at Um = 133V, a maximum frequency of 2.82 kHz is allowable. Table 2 shows the
maximum allowable frequencies at various Um voltages for the couplers. Note that these voltages apply to
non-radio voltages and frequencies. As long as the radio output power meets the requirements of IEC 60079-0
based on the equipment group in which it is operating, it is considered acceptable.
Table 2
Maximum Frequency for IIC AXF3S****AX AXN3S****AX AXFMS****AX AXNMS****AX (5.65 nF) AXF3S****JX AXN3S****JX AXFMS****JX AXNMS****JX (81.6 pF) AXZ3S0400ZX (2.16 pF) 250 V
826.2 Hz (1)
57.1 kHz
2.1 MHz
133 V
2.8 kHz
194.3 kHz
7.3 MHz
90 V
5.6 kHz
390.0 kHz
14.7 MHz
45 V
29.2 kHz
1.9 MHz
76.4 MHz
30 V
95.1 kHz
6.5 MHz
248.8 MHz
12 V
7.8 MHz
541.7 MHz
20.4 GHz
10 V
9.3 MHz
680.1 MHz
24.5 GHz
Um
(1)
Not approved for use in Group IIC applications as determined by Formula 5. AN00001-01
Hazardous Location Telemetry Equipment TM
Capacitive Barrier Evaluation
Application Note
10178 International Blvd
Cincinnati, Ohio 45246
Phone: 513-860-5465
Fax: 513-860-5464
www.solexy.net
Page 5 of 6
Example 3: Determine short-circuit current when max Um combines with radio output
When a maximum supply voltage of 250VAC @ 50Hz is combined with the output of a 1W, 2.4GHz radio, the
resulting waveform is found by adding the separate waveforms together. The output voltage of the radio can be
found as:
√ ∗ 50
√1
∗ 50
7.07
In this instance, we will call the 250VAC, 50Hz signal y1(t), and the 7.07V, 2.4GHz signal y2(t). The resulting
waveform is y1(t) + y2(t). The combination of the waveforms does not result in a 250VAC, 2.4GHz signal as can
be seen in Figure 2.
Figure 2: Combination of 250VAC, 50Hz with 7.07Vrms, 2.4GHz
Figure 2: Combination of 250VAC, 50Hz and 7.07, 2.4GHz waveforms
Because the capacitors act as a filter of varying frequencies, the short-circuit current on the output of the coupler can
be found by evaluating each of the input signals separately or by referencing the voltage-frequencies in Table 2.
Using the capacitor value of 5.65nF, the short-circuit current was determined in Example 1 to be 448µA for a
250VAC, 50Hz waveform, which falls under the curve for IIC applications. The radio output of 1W also meets the
requirements for IIC applications according to 6.6.1 of IEC 60079-0, and is therefore considered acceptable.
AN00001-01
Hazardous Location Telemetry Equipment TM
10178 International Blvd
Cincinnati, Ohio 45246
Phone: 513-860-5465
Fax: 513-860-5464
www.solexy.net
Capacitive Barrier Evaluation
Application Note
Page 6 of 6
Example 4: Examining a Complete System
A system being evaluated is using the coupler with the 81.6pF capacitance value and has the following
voltage/frequency sources:

250VAC, 50Hz mains voltage

24VDC, 15W fused switching power supply with max switching frequency of 200 kHz

Radio internal switching regulator with 5VDC output and switching frequency of 600 kHz supplying radio’s
power amplifier.

Radio microprocessor operating on a 3.3V rail with a 16MHz crystal oscillator.

Radio output of 250mW, 2.4GHz
The maximum energy is determined with a 1.5x safety factor as:
1
1
1.5 ∗
1.5 ∗ ∗ 81.6 ∗ 250
2
2
3.83μ
This is well below the allowable 50µJ for IIC applications and is acceptable.
Referring to Example 2, the maximum frequencies at the present Um voltages to give allowable short-circuit currents
using the 81.6pF capacitance are provided in Table 3. Note the radio output was omitted because it falls within the
maximum allowable power for IIC applications.
Table 3
Source
Um
I
Xc
f
Mains Voltage
250 V
8 mA
31.25 kΩ
62.4 kHz
Switching Power Supply
24 V
174 mA
137.9 Ω
14.1 MHz
Switching Regulator
5V
3.33 A
1.5 Ω
1.3 GHz
Microprocessor
3.3 V
3.33 A
.99 Ω
1.9 GHz
Table 3 shows the operating frequencies of the individual voltages fall below the maximum frequencies
allowed for IIC applications. Short-circuit currents from at or below these voltage-frequencies will not fall
outside safe levels.
AN00001-01

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