Validity Period of Type Tests for Switchboards

Background

(Courtesy: Tricolite Electrical Industries Ltd)

The validity period of type tests for switchboards are many a times debatable if it is older than five years. I am sure as an engineer you have also faced this challenge during some of your interaction with your clients. We all that power equipment undergo developmental stage before commercialization. During this stage, various testing are required to ascertain the desired functionality out of that equipment. Type testing is done for the design validation of equipment for desired functionality and compliance with the relevant standards. Required testing has been defined in equipment-specific National and International Standards.

Here our focus power equipment is Low and Medium Voltage switchboard. Once switchboard is tested at NABL accredited laboratory then question arises when we need to to do it again? In other words what is the validity period of Type Tests for switchboards? So our discussion will move around validity period of type tests for switchboards only.

We all know that there has been instances when buyer has disputed validity period of type tests for Switchboards in both Low and Medium Voltage categories. Normally the buyers’ point is, the Type Tests should not be older than five years. There are three pertinent questions:

• What should be the validity period of type tests for switchboards?
• What should be the guiding principal for its validity?
• Do the standards give any guidelines about the validity period?

What Standards Say on Validity Period of Type Tests?

Probably all the designers, quality and sales engineers have faced above stated questions at some point during client interactions. Let us discuss this issue and explore the guiding principal for validity period of type tests as none of the relevant National and International standards say anything about validity period of type tests for Switchboards. So the question arises, in absence of any guidelines how to decide on the validity period of type tests? Where from engineers and designers should take the reference? Is their any universally accepted norm for it?

The Way Forward

The only way is to go by the logical reasonings to ascertain the validity Period of type tests. So, let us first understand the factors involved in type testing of switchboards. Fundamentally product design is based on the guidelines given in the relevant Indian or International Standards and that is the reference document for Type testing and design both. Any product is said to be compliant with relevant Standard if design gets validated in NABL accredited laboratory in accordance with given guidelines for prescribed Type Tests in the same Standard.

So, to say that type tests are not valid on a particular date only if

1. Standard has been revised and hence the Type testing references or/and
2. Product design has been changed by the manufacturer compared to the earlier tested design.

But there is a catch on change in design attributes of the switchboards. If the switchboards design attributes have been changed for good, changed on positive side for the same rating or requirement then the type test is valid if Standard reference is same. This means, switchboard’s particular attribute is more than the minimum requirement as per standard. So design is not compromised compared to the earlier tested design rather it has been strengthen. This kind of verification is called design verification. If it passes the design verification then type tests validity is intact. Verification methods are described in relevant Standards in detail which can be referred by engineers and designers.

Examples

Let us understand this by an example and take the case of Temperature Rise Test. This test is being conducted to verify the current carrying capacity of the bus bars in a switchboard. We will not take up all the factors affecting temperature rise test rather we will consider only one. One of the design attributes is the cross-section area of the busbar. If a particular cross section of bus bar has passed for temperature rise test, then any change in cross section on positive side and using for the same rating will not invalidate the Type Test because higher cross section has the higher current carrying capacity.

Another example is the Power Frequency Test for which reference design attribute is phase to phase and phase to earth clearance. So here again if we are changing the clearances on the positive side i.e., increasing the clearances, will make the Switchboard even better from Power Frequency Withstand point of view. But if same is revised on negative side, then we need to revalidate the design by doing the Power Frequency Type Test again. To understand the creepage and clearance refer below diagram.

Here we have discussed only a few type test requirements and design attributes as examples. For exhaustive details of type tests please refer to relevant standards.

Conclusion

Validity period of type tests for switchboards are only dependent on change in guidelines in reference standards or change in design parameters of the switchboards which can not be validated with reference to tested design. This can be followed as guiding principal for such cases where validity period of type tests for switchboards comes under question.

Frequently Asked Questions

External References