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Mccb Selection For Use With Vsd

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To introduce myself, and this thread, let me say that I am an EE who is employed by a company which manufactures/builds switchboards, and which sells the components including circuit breakers.


I was asked the other day to recommend a MCCB for use with an irrigation pump motor which was being run via a VSD.


Since the installation was connected to a rural supply with a fairly lengthy supply feeder, the choice between a 36KA and a 50KA MCCB pointed towards the 36KA.


Then the question arose as to the type of trip unit that was required. It turned out that the least-cost option was a thermal-magnetic trip, and so this was what was settled on. I have since trolled through our supplier's technical data, but I can find little about the best type of trip unit to use with a VSD, and it is this that I wish to ask about.


Our supplier recommends two options for DOL (or, presumably, star-delta) starting : either 1) a magnetic-only MCCB followed by a line contactor/thermal overload combination, or 2) electronic thermal/magnetic (adjustable) trip and line contactor only.


My thoughts are that for use with a VSD, a magnetic-only breaker is probably quite OK, but that the thermal-magnetic with the thermal trip wound up higher than the level at which the VSD is set to 'trip' at would provide a comforting measure of redundancy.


Comments anyone?

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Hi Mark


I would opt for the standard thermal trip which is there to protect the cables to the VSD. The only way to provide any measure of protection for the VSD itself, is to use semiconductor fuses in the supply, plus surge arrestors and line reactors. - All of these fall outside of the Mccb arena.

NOTE: The input current to the VSD is rich in harmonics and this will increase the effective thermal heating in cables etc, so allow for at least 30% higher current than the rated current of the VSD.


On a side issue, I have been looking at options for earth leakage breakers supplying VSDs and that is an interesting one.

A number of people are using standard earth leakage breakers and having problems of niusance tripping, but of greater concern, is what happens when you have a breakdown within the drive (after the rectifiers). The current could well be half wave rectified DC and that will not operate a standard earth leakage RCD.

There are three types of RCD, Type A, type AC and type B.

Type AC is for AC waveforms, type A is for AC waveforms and half wave rectified waveforms and type B is for the same as type A plus continuous DC. [edited 13/11/07 marke]


I suspect that type A would be adequate for the input of a VSD, but it has been suggested that it should be type B. Any thoughts??


Best regards,

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It is actually the type A RCD which is sensitive to pulsating dc. This is the type which is mandatory for use in NZ, but not in many other places in the world. Type AC is sensitive only to AC.


Turning to the nuisance tripping issue, there is a type of RCD, type Ai, which is designed to have greater immunity to nuisance tripping. I quote from the (GE) technical manual :


"Type AI and ACI (High immunity to nuisance


Electric equipment incorporates more and more

electronic components which causes nuisance

tripping to the conventional 30mA RCBOs type A

or AC (always in the most critical moment like

weekends, areas with no people presence) due

to overvoltages or high frequency currents produced

by atmospheric disturbances, lighting equipment

(electronic ballasts), computers, appliances,

connections to long cables which induce a high

capacity to ground, etc.

Some times the filter incorporated on the standard

RCBOs type A or AC which are protected to prevent

nuisance tripping against peak currents up to 25 0 A

8 /20 µ s, does not avoid 100% unwanted tripping.

Installations with lighting equipment incorporating

electronic ballasts or computers.

The most typical problem in these installations is the

tripping of the RCD when switching the equipment

ON -OFF . It is recommended that, in case several

devices are installed in the same line, the sum of all

leakages shall not exceed 1/3 ln since any

disturbance in the line can trip the RCD. For this kind

of installation it is recommended to split up circuits."


I take it that although VSDs are not explicitly mentioned here, they share similar characteristics to switchmode devices such as electronic ballasts and computer PSUs.


Consequently, I am inclined to recommend the type Ai RCD as the best solution to the problem of nuisance tripping on VSDs.


Note that there is also the type Si which are designed for use where selectivity is required, ie where you have one RCD at the head of several branch circuits, each of which is also protected by its own RCD. These have the same high immunity to nuisance tripping as the type Ai, but they have In = 300mA. If the type Ai were found to be too sensitive, this type Si might do the trick.


Best regards,

Mark Monson

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Yes you are correct, I have swapped the AC and A!


Type AC is for AC waveforms only, type A is for AC and pulsing DC components


I will edit the original posting to correct this.


Best regards,

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