SI-51 Differential Probe

	Question 1:
		The specifications of the high voltage differential probes’ input voltage seems ambiguous. It is quite confused what voltage can be measured safely and correctly. Is there any further explanation on it?

Answer 1:

　As encountered for many customers, question arises time and again regarding the specifications of the input voltages. The follows is an example (SI-9010) of the ambiguous part which often confused customers:

Input Voltage:
  Differential Range *
    + 700V (DC +peak AC) or 700 Vrms @1/100
    + 7000V (DC +peak AC) or 5000Vrms @1/1000
  Common Mode Range*
    + 7000V (DC +peak AC) or 2500Vrms @1/100 & 1/1000
  Absolute Max Voltage*
    + 7000V (DC +peak AC) or 2500Vrms @1/100 & 1/1000 in common mode
    + 7000V (DC +peak AC) or 5000Vrms @1/100 & 1/1000 in differential mode

  *Voltage limit is the lesser of the DC + peak AC and RMS values

In order to clarify the ambiguity, a table and two figures are presented as following:

Table 1. Explanation of Ambiguous Specifications

Clause 1 Input Voltage

Explanation 1 This section specifies the range of voltage which appear between + input and - input of differential probes or between each input and ground. We suppose the input voltage is a working voltage superposed with transient overvoltages. The working voltage could be in different forms such as dc, sine-wave, square wave, pulse, train, …etc. and the transient overvoltage could be an impulse such as a 1.2x50μS impulse.

Clause 2 Differential Range* +/-700V(DC+peakAC)
or
700Vrms @1/100

Explanation 2 Differential range means the operational range of the differential input voltage (i.e. the voltage which is across + input and - input). The differential input voltage must not exceed this range. Otherwise the output of the differential probe could be distorted and incorrect measurement could result. The total peak value of the working voltage and the transient overvoltage must stay within the range (between +700V and - 700V). The rms value of the working voltage must be less than or equal to 700V. At the condition of 1/100 attenuation ratio.

Clause 3 +/- 7000V(DC+peak AC)
or5000Vrms @1/1000

Explanation 3 The total peak value of the working voltage and the transient overvoltage must stay within the range (between +7000V and - 7000V). The rms value of the working voltage must be less than or equal to 5000V. At the condition of 1/1000 attenuation ratio.

Clause 4 Common Mode Range* +/- 7000V(DC+peak AC)
or
2500Vrms @1/100 & 1/1000

Explanation 4 Common Mode Range means the operational range of the common mode input voltage (i.e. the voltage which is across each input and ground). The common mode input voltage must not exceed this range. Otherwise the output of the differential probe could be distorted and incorrect measurement could result. The total peak value of the working voltage and the transient overvoltage must stay within the range (between +7000V and - 7000V). The rms value of the working voltage must be less than or equal to 2500V. At the condition of 1/100 & 1/1000 attenuation ratios.

Clause 5 Absolute Max Voltage* +/- 7000V(DC+peak AC)
or
2500 Vrms @1/100 & 1/1000 in common mode

Explanation 5 To exceed this limit could cause the differential probe damaged or deteriorated. The total peak value of the working voltage and the transient overvoltage must stay within the range (between +7000V and - 7000V). The rms value of the working voltage must be less than or equal to 2500V. At the condition of 1/100 & 1/1000 attenuation ratios. The voltage specified is the common mode voltage.

Clause 6 +/- 7000V (DC + Peak AC)
or
5000 Vrms @1/100 & 1/1000 in differential mode

Explanation 6 The total peak value of the working voltage and the transient overvoltage must stay within the range (between +7000V and - 7000V). The rms value of the working voltage must be less than or equal to 5000V. At the condition of 1/100 & 1/1000 attenuation ratios. The voltage specified is the differential mode voltage.

Clause 7 * Voltage Limit is the lesser of the DC + peak AC and RMS values

Explanation 7 In other words, both specifications of the DC + peak AC and RMS value must be satisfied simultaneously.

		The following figure 1 and figure 2 are extreme examples which are complied with above specifications.

	Question 2:
		The SI-9010 input wire sets are marked 1000V CATIII double insulation. How can they be used at common mode voltages up to 7000V (DC + peak AC) or 2500Vrms? And, how can they be used at a differential voltage up to 5000Vrms?

	Answer 2:
		According to IEC-61010-1, the test voltage for dielectric strength of 1000V CAT III double insulation is 6800Vac (by table 8 and table 9), while that of 7000V (DC + peak AC) 2500Vrms CAT I basic insulation is only 4800Vac(by table 6 and table 9). In other words, the dielectric strength requirement of 1000V CAT III double insulation is stricter than that of 7000V (DC + peak AC) 2500Vrms CAT I basic insulation. Therefore, the input wire sets marked 1000V CAT III double insulation can be used at 7000V (DC + peak AC) 2500Vrms CAT I basic insulation. As to a differential voltage up to 5000Vrms, it is an extreme case that both inputs of SI-9010 are connected to signals of 2500Vrms working voltage and 180º phase difference. By the way, we would like to repeat the explanation in Answer 1, i.e. 7000V (DC + peak AC) 2500Vrms means the signal whose total peak value of the transient overvoltage and the 2500Vrms working voltage is 7000V.

	Question 3:
		Does IEC-61010-1 give clear statements on the allowed voltage for 1000V CAT I, 1000V CAT II and 1000V CAT III?

	Answer 3:
		IEC-61010-1 give clear statements on test voltage of dielectric strength for working voltages at different measurement categories (by table 6, table 8 and table 9).

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