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What is being measured ?
Figure 1 below is a rough schematic of a typical line operated instrument with patient connections. All measured parameters are labeled for discussion. All these measurements are made with the DALE600/600E Electrical Safety analyzer. In reference to the LT544D-20/20E Digital Safety Analyzer, the same circuit applies, with the exception of the "ISOLATED PATIENT CIRCUITS" and it's associated parameters.
Line Voltage [VL] is the mains power, supplied by the electrical distribution system of the institution. It is shown as aa three (3) wire system of HOT, NEUTRAL and GROUND, with NEUTRAL like GROUND returned to true Earth at their entry into the building. The voltage measured will depend upon:
Instrument Current [IL] is the surrent used by the device under test.When turned on, the device should be operated in its various modes to determine the worst condition to track. Measurement is made in the HOT wire via transformer coupling to ensure that the total current is measured as it is possible that the NEUTRAL and GROUND wire could share the return path.
Chassis Resistance [RG] (grounding resistance) is the resistance from the device's conductive grounded chassis to the grounding terminal on the receptacle in which it is plugged.The resistance is largely made up of the GROUND wire in the power cable, and directly proportional to its length. The resistance measurement also includes the junction resistance in connecting the wire at both ends and the bulk resistance of the chassis from the grounding point to the point of measurement.
Underwriters Laboratory (UL) limits the ground resistance to 100 milliohms for new products and the National Fire Protection Association (NFPA) to 150 milliohms, 500 milliohms for devices in the field. Maintaining a low resistance is important to protect against the chassis becoming "hot" as a result of an internal fault causing current to flow to the chassis. The resulting voltage drop across the ground wire will raise the potential of the chassis with respect to the local ground, and thus create a potential hazard.
Earth Current [IE] (internal chassis current) is the current that flows in the GROUND wire of the power cable that returns the chassis leakage current to the true earth ground. This current does not constitute a hazard as long as the ground wire remains intact, and the current does not become excessive. This could occur by a major fault, resulting in the ground wire sharing the load current with the neutral wire, or supplying the total return. It should be pointed out, that this current becomes the chassis leakage current for conductive enclosure when tested under open ground condition as discussed below.
Leakage current is due to the proximity of the hot wire or the line potential components to the chassis, represented by ZL a combination of capacitance CL and resistance RL components. Particular sources of this current:
Measurement is made using a 1000 ohm AAMI load placed directly in series with the ground wire. Open neutral will usually represent the worst case.
Figure 1. Block diagram of a line operated instrument with isolated circuits.
Chassis [Enclosure] Leakage Current [IC] ,also referred to as external leakage current is that current that flows between the accessible conductive chassis or enclosure, and earth ground. As stated above, this current under the condition of an open ground is the same as the earth current. With ground intact, the current should be very low, reflecting the miiliohm impedance of the ground wire paralleling the 1000 ohm AAMI load.
Differentiation is made between Earth and Chassis current due to the wide use of insulated enclosures today, and thus no accessibility to conductive chassis. Under these conditions, the Earth current represents total leakage current, while the Chassis current measured with a 14 x 14 cm piece of foil placed in intimate cocntact with the enclosure provides indication of potential hazard due to contact. Capacity between the chassis and the foil will determine the current measured, which usually will be low.
Lead To Ground [IP] (patient source) current is that current that flows between an individual patient lead and ground. It represents the condition of a patient with leads attached touching ground, such as an electric bed. If the patient connections are not isolated, this current will reflect the earth current, when tested under the open ground condition as this is it's only path back to true earth.
For devices incorporation isolated patient connections, this current us reduced by the patient isolating impedance, ZI, a combination of resistice and capacitive leakage. Measurement is made to the lead selected by the lead selector switch with the other side of the AAMI load connected to system ground. Current measured should be the same for all leads, including the ALL position, as the current represents the isolation impedance to the patient circuit.
Lead-to-Lead [IA] (auxiliary current) is that current that flows from one lead to another, and includes the following:
The current can be DC or AC, or a combination of both. Therefore it needs to be measured utilizing true RMS technique. This method avoids the wave form introducing errors in converting th an RMS value, since each wave form has their own conversion factor. The DALE600/600E measurements are made with a true RMS converter to provide the common base necerrary for accurace readout with a variety of common wave forms.
Measurement is made with a completely floating circuit to avoid extraneous leakage currents to ground introducing errors. Furthermore, these measurements are made from the selected lead to all other leads connected together as this represents the worst case and to reduce the permutations required to cover all possibilities. The single lead carrying the most current will in general be the reference lead, RL as this acts as the return for the other leads. For this reason, to obtain a true reading of the reference leads in a 10 lead system, a C lead adapter is required to extend the chest lead capability.
Lead Isolation [II] (patient sink) current is that current that would flow into the device undertest, if the patient were to come into contact with full line voltage. This could occur again with an electric bed, but which has become ungrounded, and has a short to the frame. While measurement is made in each individual leads, a common value will be found for all leads as well as for the ALL position, as this is the measurement for the isolation of the patient circuit. To assure proper reading, test should be run with ground intact. (For this test, the DALE600/600E provides a specially shielded line voltage secondary on its power transformer that minimizes internal leakage currents from the measuring circuit, and thus avoids the necessity of measuring that current, and subtracting it from all readings. The current is limited to approximately 1 mA for user protection, and for additional safety, only applied to the patient leads when the TEST switch is pressed to ISO).
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