From Product Safety UL 60950, "Safety of Information Technology Equipment," defines numerous safety tests for information technology equipment (ITE), including electrical shock, radiation hazards, chemical hazards, wiring, physical construction, connection to telecommunication networks and flammability. Per UL 60950, this standard is "applicable to mains-powered or battery powered information technology equipment with a rated voltage ≤ 600V and designed for installation in accordance with NFPA 70 and CSA C22.1, CSA C22.2 No. 0" [1]. Examples of applicable equipment include cash registers, copy machines, modems, ATMs, personal computers, postage machines and typewriters.
If you have read (and re-read) paragraph 2.6.3 of UL 60950 and are still confused as to which current to use in the ground bond test, you are not alone. The intention of this article is to investigate, define and clarify the protective earthing and bonding requirements in paragraph 2.6.3.3.
Ground Definitions Let's first define the terms used in ground bond testing and those found in UL 60950.
Ground Bond Specifications UL 60950, paragraph 2.6.3.3, “Resistance of earthing conductors and their terminations,” states: 1: Protective Earthing (PE) Conductors comply without test. 2: Protective Bonding (PB) Conductors are divided into two groups. 2a: Protective Bonding (PB) conductors that comply with the minimum conductor sizes specified in Table 3B and are terminated as specified in Table 3E, comply without test. 2b: Those PB conductors that do not, are subjected to the following resistance test:
Test Time (minutes) ≤30 2 30 ≤ 60 60 ≤ 100 100 ≤ 200 >200
Table 1: Test Time for Current Rating > 16A
The resistance of the PB conductor shall not exceed 0.1Ω.
Before calculating an example, investigate what the requirements for protective earthing really entail for the device under test (DUT). Over-current protection, circuit classification, test current and current rating are defined in the following paragraphs to further clarify the PE test and DUT.
Figure 1 illustrates over-current protection between the person touching the device under test and the possible fault current in the device. There is over-current protection between the AC mains supply coming into the facility and the electrical device under test. The DUT has two levels of protection in the form of fuses and electrical insulation between the live circuit and the operator. So that, in the case of a fault current, the resistance between the operator and DUT is ≤ 0.1Ω.
Figure 1: Over-Current Protection
Classification of Equipment The equipment under test, specifically the part of the equipment to be tested (circuit requiring PE) must first be classified in accordance with paragraph 2.6.1 of UL 60950. Paragraph 2.6.1 discusses what types of current the earthed part is likely to carry, the use of over-current protection and the use of protective earthing for different purposes in SELV and TNV circuits and other parts. All parts likely to carry fault currents intended to operate over-current protective devices are specified in subparagraphs a, b, c or d. Earthed parts that carry other currents, NOT intended to operate an over-current protective device but are required to be earthed, are specified in subparagraphs e, f and g.
Parts likely to carry fault currents:
Parts likely to carry other currents:
The equipment (specifically the circuit requiring PE) can now be placed into a test category. Based on what current the circuit is likely to carry, the test current for the ground bond test can now be selected.
Current Rating of Device Under Test Test Current for devices specified in:
The circuit is classified and the test current is selected; now, determine the current rating for the circuit under test.
Current Rating Current rating of the circuit under test can also be confusing, especially when the circuit is just one part of an electrical device. The current rating of the circuit depends upon the location of the over-current protective device(s) and the current rating shall be taken as the smallest of the following conditions: 1. Rated current of the equipment 2. Rating of an over-current protective device specified in the equipment installation instructions. (Generally a 20Ampere circuit is assumed) 3. Rating of the over-current protective device in the equipment that protects the circuit or part to be earthed.
In some cases it might be acceptable to assume worst case value for current rating, which would be condition #2, and this would give a test current of 40Amperes. The standard however specifically states the “smallest of the following,” not the largest or worst case. In most conditions, either #1 or #3 above would be chosen as the current rating, assuming that over-current protective device, such as a fuse, is located in such a way that current from the primary circuit (i.e., mains) does not have an alternate path around the fuse. A device where the use of condition #2 would be applicable is a rack cabinet that has no over-current protection.
Device Under Test The device under test can take on many forms: an electronic instrument, a circuit within the electrical equipment or plug-in electrical devices. But determining the protective earthing of circuits within electrical equipment and measuring their respective ground resistance can be tricky. Let’s look at three device examples and how a ground bond test would be performed to check for protective earthing of said devices.
Take a computer hard drive tower. It has a 3-prong power cord, the 3rd wire being the ground plane, protective earth connection, for the tower itself. Inside the tower there are many plug-in components (power supply, memory card, network interface card, sound card, video card) but they are grounded by connection to the tower case. Figure 2 illustrates the most straightforward ground bond test, measuring the resistance through the power cord.
Figure 2: Ground Bond Test Setup
Basic Ground Bond Test The ground bond test in Figure 2 verifies the ground connection of the case of the DUT (here a computer monitor) through the power cord, also known as the chassis ground connection. Earthing and bonding refer to the grounding of any exposed metal on the device under test (DUT) to the main Protective Earthing (PE) terminal. Before a high voltage (hipot) stress test is done, a ground bond test is typically done to verify that the DUT is indeed grounded before the high voltage is applied. A 4-terminal Kelvin connection to the DUT is illustrated in Figure 2.
A ground bond test consists of three steps: 1. Apply high AC (or DC) current to the DUT with low voltage for a period of time 2. Measure the voltage drop between the PE terminal and the part to be earthed 3. Calculate the Resistance = Voltage drop divided by the Applied Current.
For devices without power cords (e.g., those that plug into a rack or system), the ground bond test is not so straightforward. Next, we’ll investigate two such examples, and calculate their respective test currents.
Plug-In Devices The two examples of devices (circuits) under test that we’ll determine the PE test current for are a computer power supply and a telecom carrier card.
Figure 3: Plug In Device
Computer Hard Drive Power Supply The power supply would fall under the “¶ 2.6.1.b, c, d with PE and Basic Insulation” definition of parts likely to carry fault currents. Therefore, its test current would depend on the current rating of the circuit under test, which is 2 times the current rating.
This particular power supply for a computer hard drive can provide 20A @ +5V. This current (20A) is > 16A; therefore the ground bond test conditions would be: Current = 2 X (20A) = 40A with the test voltage equal to 5V for a duration of 2 minutes. The measured resistance should be < 0.1Ω. But wait! If the power supply hot/neutral connections are fused, then the current rating of the ¡®circuit under test’ becomes the current rating of the fuse. Said power supply has a fuse rating of 5A/250V. Therefore, the ground bond test condition becomes: 2 X (5A) = 10A at 5V for a duration of 120 seconds.
Telecom Carrier Card The telecom carrier card, plugged into a slot of a multi-port telecommunications network, would fall under the “¶ 2.6.1.e” definition of parts likely to carry other currents whose power source is a telecommunications network. The test current would then be 1.5 times the maximum current from the telecommunications network (if known) or 2A, which ever is larger. A telecom carrier card is not fused but has a laminated ground connection. It is inserted into a multi-port network against a backplane.
Not knowing the maximum current of the TN, calculate the test current using the current rating of the telecom card. The telecom card itself provides 2A @ 5V. This current rating is < 16A, so the ground bond test conditions would be: Current = 2 X (2A) = 4A with the test voltage equal to 5V for a duration of 120 seconds. The measured resistance should be < 0.1Ω.
An Additional Note on Ground Bond Testing The Canadian Standards Association’s document CSA 22.2 No. 0.4-04: “The Bonding of Electrical Equipment” calls out a 40A ground bond requirement in paragraph 4.1.2.2. This requirement states that “the current used for the impedance measurement for cord-connected equipment be twice the rating of the attachment plug but not less than 40A.”
CSA 22.2 No. 0.4 divides the bonding test into three parts: impedance, continuity and short circuit. The continuity test verifies that a fault path exists between any exposed conductive metal surface and power line ground. The impedance test is the application of high current to verify the strength of the bond. The short circuit test is employed if a fault is detected in the ground circuit and is then used to measure the extent of this fault.
CSA 22.2 No. 0.4-04 defines bonding as “a low impedance path obtained by permanently joining all non-current carrying metal parts to ensure electrical continuity and having the capacity to conduct safely any current likely to be imposed on it.” Translated into simpler terms, bonding is the safe current path from exposed conductive parts to power line ground that is capable of safely handling all plausible current.
CSA 22.2 No. 4 applies to electrical equipment installed and used in accordance with the Canadian Electrical Code (CEC). The electrical equipment is specified as either “(a) cord connected or permanently connected or (b) constructed to ensure it will be bonded when installed.” If the product under test is to be sold in Canada review CSA 22.2 No. 4 requirements to verify the product’s grounding circuit meets these CEC requirements. g
References 1. UL 60950 ¶1.1.1 Equipment covered by this standard. 2. UL 60950 ¶1.2.13.11 and ¶1.2.13.10, respectively 3. UL 60950 ¶1.2.8.2, 1.2.8.3, 1.2.8.5, 1.2.8.6, and ¶1.2.8.9-12, respectively
About the Author Tracey Gilpin is a Technical Writer with QuadTech, Inc., and can be reached at tgilpin@quadtech.com. © Copyright 2007 Conformity |
