Equipping the LVD Test Laboratory

Introduction

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Compliance Proving or Production Test

To aid this potential for confusion, when we look at the specifications for the various equipment we see that there are a wide range of options. It is therefore vital that we understand the essential characteristics and properties of each: otherwise, it is quite likely that we will not purchase the 'best' options for money. Therefore we will carefully consider what will influence our choice of equipment.

We shall therefore consider what is essential, and what characteristics it would be 'nice - to - have'.

Classifications of Requirements

As we know - all mains powered equipment should have an Earth Bond - except, obviously Class II - and HiPot test. (Yet it is quiet alarming to realise how many manufacturers fail to implement these basic, rudimentary, safety tests.)

Earth Bond

It is useful to have some form of "Zero-Calibration" so that when the probes are short-circuited the unit reads zero but we can always record the numbers and do the sums!

We must remember that these small, lightweight Earth Bond testers have limitations - usually they will require a 30 second "rest" between measurements - so that the transformer can cool - but otherwise I have found them (generally) satisfactory.

Do, however, be aware that some of the 'clever' testers take charge of the test. Some will indicate "Failure - High Earth Resistance" - without giving a reading! Others just tell us "Pass" or "Fail" - I have a preference for numbers.

A good manual or semi-automatic Earth Bond tester will be quite satisfactory for Production Testing or Type-Testing.

As production quantities increase we need more specialist products. Bigger transformers capable of high duty cycle and a computer interface for control or data logging.

One client had a problem with lead resistance but was operating full statistical controls - this enabled them to remove products and test them on their type-test equipment so that they could re-calibrate their production (statistical) 'control-line'.

HiPot

For Production voltages between 1,500 and 2,500 Volts are necessary to verify that insulation is in place. Anything less may "Pass" faulty insulation.

For Type-Test it is important to read the standard - test voltages of up to 4,500V may be required.

Some standards allow AC or DC HiPot testers. DC testing should always be the preference because measurements are not affected by filter capacitance - but do make sure there is a indication that the external load is discharged after testing. A 0.1µF capacitor charged to 2,000V stores 0.2 Joules and can pack an nasty punch to the unsuspecting user.

Other standards require 250VA (or higher) output to produce a "burn out" test. If specified then the correct power must be used.

High power is useful for tracing insulation faults but is not the only method. Some of the better 'Intelligent' HiPot testers have built-in sensing and will ramp continuously to provide a series of low-power flash-overs. This achieves the same result but with much less power. My personal preference is to use the lowest power possible (5-10mA) for reasons of 'self preservation' and limiting potential damage to circuits.

Mechanical Test Equipment

They are not easy to procure - in fact - I know only of one company from where they - plus everything else you will need - can be sourced. (Details at end of this article.)

Force Probes to produce the 30N and up to 250N force tend to fall into two classification:

• Dial-gauge - designed for high use
• Probe-type - designed for low use

Test Boxes

The following describe some of the 'break-out' boxes that can be used - some are identified as 'nice-to-have' others as 'essential'. In brief these are summarised as follows:-

• The Constant Voltage Transformer (CVT) - optional - to stabilise the units connected downstream
• The Inverter - essential if equipment is to be used with a different mains frequency
• The Variable Transformer - essential
• Earth Leakage - essential
• Rise of Resistance - optional for measuring the temperature of wound components
• Outlet box - optional it makes life simple by having a range of UK, European, US and other outlet connectors

The Constant Voltage Transformer

The Inverter

It must be protected from instantaneous and permanent short circuits and open circuits. This device can be subject to high fault currents and inadvertent misuse, and needs to be 'bomb-proof'.

It will be used for a variety of tests, including thermal, and must be permanently rated for the maximum load.

The Variable Transformer

Special note to North American engineers:- You will not make true measurements if you derive 230 volts between two 115V phases. The differential voltage will be correct but the voltage with respect to ground will be only half that of the EU and you will measure about half the 'true' Earth Leakage Current.

The Variable Transformer break-out Box (Essential) takes an input of 230V 50Hz or 110V 60Hz and allows the user to control the output voltage up to 10% above nominal mains voltage.

The mains feed is broken to allow voltage, current or power to be measured reliably and safely.

Earth Leakage Box

There are three basic methods of measuring Earth Leakage Current (ELC) but the standards only accept measurements made by two of them.

The first method involves using an isolating transformer, to isolate the Safety Earth Ground (SEG) from the equipment. The second method disconnect the SEG using a switch leaving the EUT floating. Both methods measure the current flowing through a precision shaping (filter) network connected to the 'floating' circuit. The third method uses a current transformer in the SEG line.

Whilst there are distinct advantages to this third technique the use of current transformers is not recognised by most of the safety standards and therefore we do not use them for Type-Testing.

Of the two acceptable methods the second method is quite common because it produces a light and portable method of test. (Note An isolation transformer capable of 3kVA is seriously heavy!) The ELC test (along with most of the other Type - Tests) is commonly performed in a small cordoned-off area.

The filter network in these test sets is extremely venerable to catastrophic damage (I have destroyed several networks when measuring faulty equipment.). Also, some standards may require a different shaping filter. Therefore it is wise to select test equipment that can be fitted with a range of filters.

Rise of Resistance Box

Outlet box

Documentation

We will also need to produce instructions that detail what documentation will required from our engineering teams.

Test Methods

If this level of documentation is not supplied then we must produce detailed instructions to enable these - potentially lethal - items to be used safely by our Compliance Engineers.

(Please note that Compliance Engineers are not Test Engineers nor are they Design Engineers but a very rare blend of the two.

Test Reports

All of the major test houses and laboratories use a specialist format for test reports - these are commonly referred to as the "Competent Body" style.

These reports will reference each and every clause of the relevant standard. It will contain a brief synopsis of the condition, leaving space for the test engineer to enter their conclusion. (An abstract of one of these reports can be downloaded from http://www.gkcl.com.) It is essential to understand why there are so many individual standards.

There are so many standards because:-

• the types of hazards are so varied;
• the criteria of acceptability are so varied and;
• the levels of acceptable risk vary between application.

For a free reprint of this series of articles, or for price and delivery details of the Complete Compliance Set please contact:-

Ian Reeves
Claude Lyons Instruments Ltd
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Herts UK
EN8 7LR

Telephone: 01992 768888
Facsimile: 01992 788000
Technical enquiries to:-

Gregg Kervill
G.K. Consultants Limited
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