Withstand voltage

[jdd]

What is the withstand voltage for a typical consumer unit?
(eg - Wylex 10 way board to BS EN 60439-3)

Is the withstand voltage a minimum or maximum?

Your valuable comments are appreciated...

 

[jdd]

I should have also have said, I think it is 16 kA or 16.5 kA but can't find it in black and white anywhere.

 

[trev]

Voltage is measured in kA now?
What is it you're trying to prove or find out mate?

 

[telectrix]

think a bit of confusion here. the withstand fault current is, i think, 16kA. a withstand voltage, IMO, would be the max. voltage that could be applied between L and E before a flashover or breakdown of insulation occurred. and i ain't got a clue what that value might be.

 

[Des 56]

Have a read of chapter 44 Bgb, and why are you quoting amps when asking about voltage

Where there are overhead lines ,this would apply
For distribution boards where the nominal voltage of the installation is 230/240V or 277/480V category III, 4kV would be appropriate.

 

[Engineer54]

4KV DB Flash testing is quite common in many European countries...

 

[telectrix]

you get locked up here for flashing.

 

[Guest123]

Here's one I made earlier

Annexe Za of BS 60439-3

Part 3 of the Standard, BS EN 60439-3, deals with the particular
requirements for low voltage switchgear and control gear​

assemblies intended to be installed in places where unskilled​

persons have access to their use. This covers the supplementary​

requirements for enclosure distribution boards suitable for indoor​

use containing protective devices intended for use either in​

domestic applications or other locations where unskilled persons​

have access. Control and/or signalling devices may also be​

included.​

These distribution assemblies are for use on AC supplies, with a​

nominal voltage to earth not exceeding 300V. The outgoing​

circuits include short circuit protective devices, each having a​

rated current not exceeding 125A with a total incoming load​

current not exceeding 250A.​

In the UK, such equipment is referred to as a ‘Consumer Unit’​

and as such is covered by this Standard. However, additional​

requirements from annex ZA of the standard call for the​

assembly to have an additional test; this is known as the​

‘conditional withstand test’. The condition is that the consumer​

unit must withstand a 16kA short circuit fault when protected by​

a 100A HRC fuse to BS 1361 type II. Further requirements are​

addressed by this annex:​

• ​

Means of isolation to be via a manual double pole switch​

disconnector​

• ​

Rated current of consumer unit is determined by current​

rating of incoming device.​

• ​

Outgoing protective devices can be MCB’s, Fuses and/or​

RCBO’s.​

"​

 

[Knobhead]

In the absence of the OP we don’t know if it’s KV or KA we should be talking about.
Lenny’s covered the KA side so I’ll go for the KV side of things.


Test Conditions
Two distinct forms of testing are usually recognized: type testing and production line testing.
Type-testing levels vary according to the relevant product-specific standard. Class I equipment normally requires between 1000 and 1500 V applied for 1 minute with a trip level of up to 100 mA. Some standards specify that "no flashover shall occur," suggesting that the trip level should be 0 mA. But, because all devices have a small amount of leakage, 0 mA is not practical. A low level of between 3 and 5 mA is usually selected. A higher level should be used if the device under test (DUT) has large capacitive leakage.
For Class II equipment, voltages are usually between 2500 and 4200 V, but with timing and trip settings similar to Class I equipment. The same criteria for the fault-trip levels are applied to both Class I and Class II products.
Production line hipot testing requires special considerations in terms of conditions such as duration and voltage levels. On the production line, the need to have faster, but equally rigorous, tests is addressed by applying 10% overvoltage, but reducing the test duration to a few seconds. Therefore, a type test with a voltage rating of 1250 V would be carried out at 1375 V on the production line, with trip levels of 5 mA. Often, the annex section of a standard advises routine testing. When a recommendation is not available, the manufacturer must apply a suitable test-voltage level to ensure that the device is safe to be offered for sale.
One note of caution is that it is possible to get an apparently satisfactory result when the equipment under test is switched off or not properly connected. It is imperative to ensure that the equipment is switched on and properly connected. Even for experienced operators, this can be a challenge. In a production line situation, such problems are greater because of the greater throughput of products. Solutions include:

• A simple continuity test, applied on live and neutral, built in to the test program prior to the flash test.
• The detection of capacitive leakage that occurs whenever an ac hipot is applied. If no leakage is detected, a warning is initiated.
• Regular fault simulation at the test connection point.

Pass-Fail Criteria. The test itself is not quantitative, and "fail" is recorded if a breakdown of insulation or a flashover between components occurs. Most testers indicate pass or fail via a warning light or sound that activates when 5 mA of leakage occurs.
Hipot and Insulation Testing. On first examination, these two tests appear very similar. However, hipot testing is designed simply to detect gaps or clearances between conductive parts and earth, pinholes in insulation, and other degradation. Insulation resistance testing is designed to provide an actual quantitative measurement of the insulation quality.
If a wire were positioned 1/2 mm from exposed metal, an insulation test—conducted in dry air—might easily provide a pass reading. A hipot test, however, is more likely to detect this situation as dangerous. Similarly, if insulation were somehow contaminated, a hipot test would produce a pass, whereas an insulation test would highlight a deficiency. For example, the normal minimum insulation resistance value for Class I appliances is 2 M(omega). With a 1500-V hipot test, the current would be 0.75 mA and would not be detected by the 5-mA trip that must accommodate the capacitive losses that occur. Obviously a dc hipot test with a leakage meter can provide insulation resistance monitoring because the capacitive component is overlooked after the initial inrush.
Test Duration. The best way to maximize productivity is to minimize the time taken to apply all of the safety and functionality tests. By using an integrated test station and enclosure connection to the DUT, only one test sequence is required. Establishing a connection is often the most time-consuming part of production line testing, so combining four or five tests at an integrated test station can significantly reduce test times.
Production Line Safety. Type tests often call for high levels of high voltages to be applied for up to 1 minute. This is not practical in production facilities. In many facilities, a 1-minute test would adversely affect productivity. The call for a 100-mA trip level can be potentially lethal. In addition, voltage levels and test procedures realistically demand a skilled operator.
Because production line hipot testing is conducted with reduced test times, it reduces the risk to operators. Effectively designed test instruments mean that the required operator skill level can be reduced, and the use of high trip levels can be protected by a key system for which only qualified operators have access.
Safe Test Areas. With the integration of electrical safety testing standards in EN 50191, specific safety conditions have been specified for all locations where electrical testing is carried out.[SUP]1[/SUP] For example, the use of test enclosures on the production line is advisable to maximize the safe working area around the points where flash tests are to be applied.
The type test, with its high-level leakage limits at 100 mA, is potentially lethal to the human body. Hence, type testing is carried out in a laboratory and not on a production line. The test is also only to be carried out by a skilled person who is aware of the potential hazards and who is following procedures clearly defined before any test is applied. The ideal situation is for the DUT to be enclosed in a safety-test enclosure with automatic isolation of the test points on the enclosure opening. This enclosure protects the test technician from electrical sources as well as from airborne particles caused by an unforeseen failure of the DUT that terminates in an explosion.
Class II Equipment
In Class II equipment, the absence of an earth requires protection via primary and secondary insulation. Hipot testing of Class II equipment involves much higher voltage levels, typically between 2500 and 4200 V. A common problem, particularly on new equipment, is that failure can be detected on the primary insulation that is undetectable by a hipot test on the outer surface, which tests the secondary insulation only. Testing programs must include both tests.
To test the primary protection, the selected method must access the primary insulation. This is essentially a contradiction in terms, because this connection needs to be inaccessible metal. However, experience shows the following options are feasible:

• Test the primary insulation prior to final assembly. Be sure to check that on assembly no degrading of this protection takes place (e.g., screws penetrating the insulation).
• Design the product with an access that can be permanently sealed after testing. This is often an element that product designers fail to anticipate.
• Design test jigs and probes that allow access through the enclosure, ensuring that the integrity of the product (in terms of the relevant standard finger tests) is maintained.

Testing also needs to be carried out on the secondary protection. Standards generally require that the product be wrapped in aluminum foil so that high voltages can be applied to all outer surfaces. This test may be practical for laboratory situations, but it is impractical for production testing, because of both the complexity in test setup and time required and because the outer surfaces of the product must be easily marked. The use of conductive foam in a special jig creates a nest or envelope around the outer surface of the product; test voltages can then be applied. Although this method is not quoted in standards, the standards authorities recommend this procedure.
Does Hipot Testing Degrade Insulation? The view that hipot testing is essentially a destructive test is often an area of discussion. This view originates from the use of flash in type testing where the long time period required provides potential for the degradation of insulation. However, in terms of production line testing, the reduced time period and the 5-mA trip setting significantly reduce this risk. The fact remains that many manufacturers successfully conduct the test without witnessing any degradation. Under certain circumstances, an ac flash test could corrupt sensitive electronic components.
Sensitive Equipment
In situations in which ac hipot testing could corrupt sensitive electronic components, the following solutions are possible:

• Use a dc hipot test. The voltage must match the specified peak ac voltage, which is achieved by multiplying the specified ac voltage by 1.414. A discharged facility following application ensures that no residual voltage remains.
• Use a soft dc hipot test. This test requires ramping up to the required voltage. In some instances, this test can benefit from ramping down as well. This process involves a slow ramp up from zero to the required value, and then holds for a timed period before ramping back down to zero and discharging the unit under test.

Suppression Devices
The advent of measures to control EMC has increased the use of suppression devices, but these can cause problems for hipot testing. It should be noted that most designers of such components have upgraded their products to meet the specified tests. However, in some cases the following solutions may be necessary:

• Disconnect such components. Some standards allow disconnection for safety testing. However, it is important to note that disconnection is often impractical for production line testing.
• Set higher trip levels (e.g., 10 or 15 mA). The use of this option should always be accompanied by the use of safety precautions such as key-locked switches, thereby ensuring proper authorization to conduct the test. The safest solution in such circumstances is to conduct the test with the test item housed in an enclosure and with appropriate interlocks. Such precautions do not need to be complicated or expensive to provide maximum operator safety.
• Apply the high-voltage test from a dc voltage source. Almost all standards now provide for this option. The test voltage is the ac level times 1.414 to provide the suitable dc test voltage. A dc voltage will ignore the capacitive leakage levels.

Conclusion
The hipot test is designed to indicate whether a product remains safe when subjected to high voltage and whether the user is protected from danger. Being able to prove that electrical or electronic products comply with the various standards is particularly important if a subsequent failure or fault is identified.
To demonstrate that a product has been tested properly, manufacturers must provide sufficient documentation. Many testers now automate the testing process on the production line and retain the results. Documentation through automated hipot testing maximizes protection from liability and provides effective proof at the end of the manufacturing process that a product is safe.
Reference
1. EN 50191, "The Erection and Operation of Electrical Test Installations," European Committee for Electrotechnical Standardization (CENELEC), Brussels, 2000.

 

[Johnny-G]

This thread makes my head hurt a little bit so I'm going to click <--

 

[jdd]

Thanks for your replies, I have been out all day hence why not replying earlier. Had a bit of a brain fart earlier I did mean current. (Serves me right for not proof reading what I have typed!)

The reason for my original question was to determine if it is acceptable to have a fault current of say... 9kA with MCB's only rated at 6kA, in an enclosure to BS EN 60439-3 with a BS 1361 II B 100A 33kA at source. I am sure it is but couldn't actually find where I'd referenced it, hence I thought I'd ask you guys. (& gals)

 

[kingeri]

You could have saved Tony all that typing! But I'm glad you didn't, cos I enjoyed reading it!