SHORT CIRCUIT WITHSTAND is the ability of your switchboard to handle a short circuit fault for a specified duration. In the event of a fault, the busbar and cable supports must hold the conductors in position, and not sustain any damage, until the fault is cleared. The rating is determined without an upstream short circuit protective device being in circuit.
When it comes to the short circuit rating of your switchboard, the issue is not so much the duration (i.e. 1 second or 3 seconds) but the rated “Peak Withstand Current (Ipk)”. The damage occurs in the first cycle of the fault where the highest asymmetrical component exists.
In the following video you will see the amount of force present during a Short Circuit Withstand test. In the preliminary test, the cable broke free of the cable ties upon initiation of the fault when most of the force is present. This constituted a failed test.
In conclusion: It is important to ensure that the prospective r.m.s. value of the short circuit current (Icp) is less than the rated short time withstand (Icw) of your switchboard, but it’s also important to check that the switchboard rated peak withstand current is greater than the peak value of the short circuit current (Ipk) at that location.
JPR has successfully tested cable bus systems to withstand a 20kA for 1 second fault and bus bar systems to 63kA for 1 second.
Please contact us to find out more…….
SHORT CIRCUIT WITHSTAND STRENGTH
TABLE D.1 CHARACTERISTIC 11
Helpful points for designers
Short Circuit Withstand Strength
Ability of a switchboard to handle a fault current for a specific duration, in r.m.s.
Refer AS/NZS 61439.1 Section 9.3.2 and section 10.11
The switchboard rated short time withstand must be greater than the prospective r.m.s value of the fault current. ie Icw > Icp
The test is undertaken by bypassing any incoming Short circuit protective device (SCPD).
Tests the strength of the bus system. Refer 10.11.5.5 for pass/ fail criteria.
Peak withstand current
The associated peak current that the switchboard sees during the fault (based on power system typical impedance, X/R, ratios)
Refer AS/NZS 61439.1 Section 9.3.3, and Table 7
This is the most destructive component of the short circuit current, and is what causes the most damage, generally in the first cycle.
There are two things to consider when specifying the IP rating of your switchboard.
1. A higher IP rating may cause issues with condensation build up within the switchboard
Condensation can build up where there are conditions of high humidity, or large internal/external differential temperatures (as is the case of outdoor locations). This has the potential to cause corrosion of internal components, or worse, short circuiting causing catastrophic failure.
This can be negated with the use of anti-condensation heaters, pressure compensation plugs, louvres or vents.
2. There is generally a trade-off between higher current ratings and ingress protection
Current flowing through switchgear equipment and interconnecting bus systems generates heat. Oversizing the switchgear and bus systems (derating) can overcome this temperature rise issue but there is practically (and commercially) only so much space for oversized switchgear, or the amount of copper in a bus system that can be put inside the switchboard. In these cases, cooling becomes necessary for these higher current switchboards. Arc fault containment then complicates this as a viable solution. (more about arc fault containment in a future post).
Derating of switchboard equipment’s current carrying capacity is commonly known in the board building community. For example, in a totally enclosed compartment (without ventilation) containing a 4000A Air Circuit Breaker, the ACB terminals can exceed the ACB’s maximum operating temperature limit, thereby requiring a derating of the current carrying capacity to as low as 2500A to meet the specified temperature limits of the switchgear.
By simply providing a ventilation path in the above example, the current rating of the ACB could be increased to nearly 3000A. No matter what can be practically undertaken in the way of ventilation, the manufacture’s switchgear rating will always be derated by a certain amount when you place the switchgear in any enclosure.
The designer will need to consider maximum allowable temperature rise of the equipment within the enclosure from the ambient conditions, and the power loss (or heat dissipation) from that equipment.
AS/NZS 61439 clause 10.10.4.2 allows temperature rise “verification by calculation” for a single compartment of 630A or less.
AS/NZS 61439 clause 10.10.4.3 allows temperature rise “verification by calculation” for a single or assembly of compartments totalling less than 1600A.
For a single or assembly of compartments totalling 1600A or greater, AS/NZS 61439 requires “verification by test” or engineering “derivation from a tested design of ratings for similar variance”
JPR have undergone several tests for and have verified switchboard designs from IP40 through to IP56.
By default, JPR’s outdoor “VERICON” switchboards are built to IP56. These are fully welded and constructed from stainless steel or aluminium. JPR’s propriety IP56 vent and filter systems are covered by cowls providing enhanced protection from water but without reducing the cross-sectional area of the ventilation system’s inlet/outlet cross sectional area.
IP56 means the enclosure is dust resistant (with no harmful build-ups) and protected from powerful water jets in ALL directions.
As you can see from this video IPX6 is an onerous test requiring a jet of water from a nozzle 12.5mm in diameter, a flow rate of 100 litres / minute from all directions 2.5m – 3.0m away from the enclosure (Refer AS 60529, section 14.2.6).
JPR standard “DEMCON MKIII” modular, high current, switchboards are built for indoor environments and are constructed using modular assemblies using powdercoated and passivated zinc anneal steel parts assembled together, which means the switchboard can be easily modified to meet future needs. These standard switchboards can constructed for up to IP44, but with enhancements can be provided with IP56 protection.
AS3000 section 22.214.171.124 states that switchboards installed in an area with an automatic sprinkler system for fire suppression shall be IPX4 (unless a shield is applied). An IPX4 test is equivalent to giving your switchboard a shower, from a spray nozzle containing 121 x 0.5mm holes and a constant flowrate of 10 litres / minute from all directions 300mm – 500mm away from the enclosure (Refer AS 60529 Section 14.2.4).
When selecting the IP rating, the designer needs to weigh up what the purpose of the switchboard is, the current rating, internal temperature rise, and whether the switchboard will need to be modified in future. Whatever your needs, JPR can assist you with a switchboard solution.
AS/NZS 61439 requires dielectric testing as part of:
- The design verification process
- The routine verification of production switchboards.
Successfully verifying your switchboard’s dielectric strength is important because it ensures your switchboard’s busbar system will handle normal and overvoltage conditions. Overvoltages may occur due to lightning strikes, HV switching (transient overvoltages), or a large network load switching off (temporary overvoltages). Transient and temporary overvoltages can compromise the safety of your switchboard, however, are generally protected by surge protective devices (AS/NZS 1768) and phase failure relays with overvoltage protection (AS3000).
But there are also other reasons to verify your switchboard’s dielectric performance. The following are some common causes of failure at normal operating voltage:
CAUSE OF FAILURE
Although very rare, switchgear can be faulty out of the box. Dielectric testing after switchboard construction will pick this up.
A dielectric test will determine if the switchboard builder has unintentionally not allowed enough clearance or creepage distance between phases and phase-to-earth.
Generally, switchboard builders use AS/NZS 61439.1 Table 1 & 2 as a guide with a safety margin between their busbar supports, and if these distances can’t be attained, will use insulation to achieve compliance.
After a short circuit fault
A short circuit fault can cause:
1. Pitting of busbars resulting in a copper spray within the switchboard.
2. Busbars can warp reducing the clearance distances.
3. Protective device operation to clear a fault, can eject carbon that has highly conductive properties.
4. Protective device is damaged during the fault.
After a short circuit fault, it is recommended that a Dielectric test is conducted on your switchboard to ensure clearance and creepage distances have not been compromised and that the equipment itself is still fit for use.
It is also essential for operators to regularly inspect and clean their switchboards as part of their preventive maintenance regimes to ensure there is no dust build up over time that may compromise the creepage distances.
JPR’s switchboard technicians can assist you with your regular and breakdown switchboard maintenance needs. Find out more here…
Here are some more helpful points for designers!
CLEARANCE AND CREEPAGE DISTANCES, DIELECTRIC PROPERTIES
TABLE D.1 CHARACTERISTICS 3, 4 & 9
Refer: Informative Appendix F for some good explanatory diagrams of clearance and creepage distances
Helpful points for designers
Distance through the air between phases, and phase-to-earth
Verified by Impulse withstand test voltage (10.9.2)
If a main switchboard, minimum 6kV test impulse voltage
If a distribution board, minimum 4kV test impulse voltage
(Note: The NATA test station can perform an alternative test using a hi-pot to the same voltage level above as per 10.9.3)
Measurement by hand between phases and earth using Table 1
Distance measured along the surface between phases, and phase-to-earth
Verified by Power frequency withstand test voltage (hi-pot):
690V insulation – test at 1890kVac
1000V insulation level – test at 2200kVac
Measurement by hand between phases and earth using Table 2, but you need to know the material group (Comparative tracking index) of the busbar insulator and the pollution degree of the switchboard location
Note 1: Remember overvoltage and surge protection devices give you added protection and must be removed when performing dielectric tests above!
Note 2: Modern circuit breakers can have electronic protection equipment in them that generally needs to be disconnected before the dielectric test is performed. Always consult the manual before testing!
AS/NZS 61439 requires the verification of the strength of material parts. All these characteristics are to ensure your switchboards meet the required service life.
There are 8 characteristics within “Strength of Material parts” that need to be verified by test or by assessment.
This is a video of our Demcon modular switchboard busbar supports which has been successfully glow wire tested to 960oC. As part of the requirements if the sample catches alight it must self-extinguish within 30 seconds. You will also notice the tissue paper underneath, which must not ignite from any droplets from the sample.
We’ve also listed out the 8 characteristics below with some helpful points (Cheat Sheets) for designers!
STRENGTH OF MATERIAL PARTS
TABLE D.1 CHARACTERISTIC 1
Characteristic to be verified
Helpful points for designers
Resistance to corrosion (10.2.2)
Outdoor switchboards (made of ferrous materials) and outer hardware – Tested to Severity Test B (10.2.2.3)
Indoor switchboards (made of ferrous materials), and internal hardware of indoor and outdoor switchboards tested to Severity Test A (10.2.2.2).
Aluminium enclosures are exempt from test. Note: “In all cases hinges, locks and fastenings shall also be tested…” ferrous-based or not!
Thermal stability (10.2.3.1)
Applies only to switchboards made of insulating materials
Resistance to abnormal heat and fire due to internal electric effects (10.2.3.2)
Busbar supports and cleats glow wire tested to 960oC, all other parts inside a switchboard including earth supports tested to 650oC
For clarity, Assess means the support supplier can do the testing and give you a copy of the test report. This allows designers to swap out supports for other successfully tested supports (although make sure they are suitable for the short circuit withstand rating)
Switchgear built to AS/NZS 60947 series has been glow wire tested.
Resistance to ultra-violet (UV) radiation (10.2.4)
Powder and paint coatings on switchboards, must be adhesion tested with cutting and weather tested for 500 hours to ISO 4892-2 Method A, Cycle 1.
Note: Assess means the paint/ powder coat supplier can do the testing and give you a copy of the test report. However, the same coating procedure must be followed by the switchboard/sheetmetal manufacturer.
After testing – note the cross cuts from the adhesion test
To determine maximum size & weight of switchboard shipping section. Must be tested at 1.25 x the maximum weight.
Mechanical impact (10.2.6)
Applies to AS61439.3 only (for example residential, commercial distribution boards operated by ordinary persons).
Durability and legibility of markings – rub tested by hand using water and petroleum for 15 seconds each.