Holiday Tester Voltage Setting Calculation

Calculating the Test Voltage One of the most common questions our technical support department are asked asked is: “What test voltage should I use to test my coating?”  The answer is not written in stone

It depends on a whole host of di!erent factors from the thickness of the material to less o"vious considerations like like the weather# It is clear that you must determine the correct test voltage needed for your particular application as this will decide which type of machine you need $ou will %nd the formulae and an online tool for calculating the Test &oltage on this page 'on(t fall into the trap of thinking the formula "elow will solve all your pro"lems in calculating the test voltage $ou must also consider a num"er of oth other er fac factor tors s )se )see: e: $ou sho should uld also re read ad thi this* s* "ef "efor ore e you ha have ve an accurate idea of the requir required ed test voltage Calculating the Test Voltage

 The level of test voltage used depends on the type of fault you are trying to %nd and the electrical properties of the coating material +s a general rule the ,+-. standard /0123145 has "een found to work well in most cases This is shown "elow:  Test  T est &oltage &oltage formula formula using using thousands of of an inch:

or Test Test &oltage formula f ormula using microns:

 To help you work out the test voltage using the a"ove formulae we can provide you with a sliding Test &oltage -alculator with our products

Calculating the Dielectric Strength  The dielectric strength is the ma6imum working voltage a material can withstand without "reaking down It is normally e6pressed in &olts7mm The material manufacturer should "e a"le to supply this information "ut if not an appro6imate value can "e found using a 8oliday 'etector Calculating the Dielectric Strength

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O"tain a sample of material with a uniform thickness of a"out 9mm applied to a sheet of metal -onnect the 8oliday 'etector to the sample with the earth lead connected to the metal and the high voltage pro"e )via a pointed pro"e* to the surface of the material ;tarting with the output voltage set to minimum< slowly increase the volts until the material "reaks down and the alarm on the 8oliday 'etector sounds =ift the 8& electrode o! the surface of the material and note the output voltage /epeat this test a num"er of times on a new area of the sample at least 32mm from where any previous "reakdowns have occurred< noting the "reakdown voltage each time Take an average of the voltages and then 4>@ of that is appro6imately the dielectric strength of the material

;o now you have a value for the dielectric strength we can look at how this relates to the test voltage calculated previously It is important to check< "efore you start testing< that the test voltage you have selected is not so high that it will actually create faults in a coating

 This would rather defeat the o"Aect of holiday detection To demonstrate this let(s look at a worked e6ample Example

;ay we have a coating 3mm thick which has a dielectric strength of B522 &7mm Csing the ,+-. formula the test voltage is: We know the dielectric strength is B522 & 7mm so for 3mm the ma6imum voltage "efore "reakdown occurs is 3 6 B522 D 9222 & 7mm +gain the test voltage is calculated to "e 99<9B2 & "ut now the "reakdown voltage of  the material is 92<222 &olts )3 6 >222* This is clearly less than our test voltage and attempting to use 99<9B2& to test this coating would result in the creation of more holes In this instance high voltage holiday detection may still "e used to locate Eaws in the coating< "ut some further testing is required to ensure that this method is valid /eferring to the e6ample a"ove of a 3mm coating with a dielectric strength of >222 & 7 mm< the validation test would "e as follows: 9 3

Fake a small hole in a test piece With the electrode over the hole slowly increase the voltage until the spark Aumps the gap ,ote the voltage )which in this instance< on a 3mm coating< would "e G >222 &*  To determine the test voltage< use a value midway "etween the test voltage calculated using the ,+-. formula )in this case 99<9B2 &* and the minimum voltage determined from the a"ove test )G>222&* This works out to "e B2H2 & ))99<9B2  >222* 7 3* J>222*

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,ow make some more holes in the test piece )making sure there is more than 32mm "etween each hole*< this time at angles< and using your test voltage )in our e6ample B2H2&* ensure that it is possi"le to locate the faults

 This method of %nding the test voltage is %ne if all you are looking for is cracks in the coating )that is complete faults that go all the way through the coating to the su"strate* Indeed< many standards only require this type of fault to "e detected 8owever< with careful selection of the test voltage< it is possi"le to %nd a variety of di!erent Eaws ;ee + guide to using '- 8oliday detectors for more information