Insulation tester - some guidance please - Model Engineer

If I remember my IEE Regs correctly testing domestic circuits normally energised at 230 volts AC is to be done at 500 volts DC, three phase industrial circuits subjected to 400 volts (line to line) is done using the volts facility. You might not need the latter but most instruments will offer the facility – they'd be no use to a commercial electrician if they didn't. Which kinds of begs the question of the quality of an instrument which doesn't offer the facility.

Hengfeng supply professional and honest service.

Insulation testing is about whether the appliance under test will kill you or not, and the same goes for the continuity tester which will likely be part of the same instrument and which performs an equally important test. At least with a calibration certificate in your hand you know if the information acquired from the measurement can be trusted.

As Emgee notes, if we could have some idea of your intended use for such measurements we could offer better relevant advice.

Seasons Greetings Simon

Hi,

Some time ago I purchased a MASTECH MS insulation resistance tester (commonly known as a megger) from Ali Express and I am very pleased with it. Good specs and a good price. Some of the cheaper units did not appear to have good low insulation resistance readings on the high voltage setting if that makes sense. So on the 500 volt setting (normal for 230-250 volt countries) the readings that are critical are in the 0.5 to 5 Megohm range as anything above this is largely academic. Some of the cheaper units did not appear to be able to display meaningful readings below 2 or 5 Megohm and this made them useless for me. Recommended unit for my general usage.

Hope this helps. Cheers and Christmas greetings, StephenS.

Thanks all for your useful and thought provoking comments.

I suppose when I think about it, it wouldn't be the most frequently used tool in my armoury. But I occasionally do small mods to my domestic arrangements and it would nice to be able to say I have made 'that final check and all is ok'.

Also what seems to be popular at the moment here is two washing dryers tripping the ELCBs due to leaky heater elements. When I check these for insulation resistance with a normal Ohmmeter you seem to be able to get any figure you want up to 20Meg depending on the time of day. I was hoping that a higher test voltage would make this kind of check more reliable. (I do hate to spend money on parts when I don't have conclusive proof that they are defective…. of course its always ok to spend on tooling )

Thanks StephenS for your positive comments on the Mastech. That looks like a reasonable device.

Is it necessary to disconnect everything before using a Megger? For example, I'd have thought putting Vdc into a anything semi-conductor like a switch mode PSU risked killing it.

Rectified single phase is around 320VDC, full wave rectified 3-phase about 720VDC. So not that far from 500V or V respectively. There are nasty spikes on the mains as well. One of the EMC/EMI mains tests applies a series of spikes on top of the mains, of both polarities, at 500V, V and V to see if anything fails. The whole test sequence can take around 2 hours. Which is long time to hold ones breath and hope the equipment under test doesn't go phut!

Insulation testers will be fairly high impedance so the currents flowing will be low. Any proper mains input equipment should have transient protection on the input against the spikes. These will cope with the low DC currents.

Andrew

If you want to know if a piece of equipment is safe to energise the insulation tester is the kit to reach for, as a low voltage resistance test using a Fluke DVM as an ohmmeter doesn't cut the mustard. Other makes are available. Quite apart from the regulations, you need that high voltage for the test to be a valid representation of the real life energisation using mains supply. So while the insulation tester isn't the most frequently used bit of kit an anyone's toolbox, when you need it, you need it.

The same goes for the continuity tester. A simple ohms check won't pass 20 mA (the specified minimum test current through a short circuit) through the earth continuity conductor, and won't find loose screws, oxidised connections and the like.

So it's a specialised gadget for a very specific purpose, being the final checks to confirm that an appliance is safe to energise. By all means do the best you can with a digital ohmmeter but bear in mind these are not representative tests of the satisfactory state of the appliance, and if you are taking responsibility for someone else's welfare (don't we all?) the tests had better be done in the approved fashion.

Don't agree about digital vs analogue, digital is much more robust, holds its calibration better, is at least one or maybe two orders of magnitude more sensitive.

Testing stuff with semiconductors exposed to the test voltage is a whole different ball game, well beyond the scope of the casual user. The possibilities for ending up with a piece of equipment which you know is safe to energise but now needs repair are endless.

As Phil has it above, if you are testing motors and heaters, an insulation tester is going to be essential, not least because it's the absolute first question anyone using the equipment should (will) ask. Has it got a valid insulation test? Second question is what is the earth continuity conductor resistance. Without confidence in the answers (for which read sight of a valid test certificate for the measuring instrument) cut the plug off.

HTH Simon

FYI measuring something with an ohmmeter and getting a varying reading is absolutely typical of having water in the circuit. Did I mention you need an insulation tester?

Seriously this is very much an example of where the high test voltage of a "Megger" (other makes etc) gives you an answer you can believe. With the high test voltage of a proper insulation tester you will (probably) get a more repeatable answer, though of itself the fact that the measurement is not constant indicates that the equipment should be withdrawn from service, even though the range of readings is otherwise acceptable.

Seasons Greetings to one and all

Simon

Measuring insulation resistance with high DC (relative to normal operating voltage) is pointless if all you are trying to ascertain is whether the equipment is safe to energize. Take a typical domestic 240V installation with MCB overcurrent protection and 30mA RCD earth fault protection. To operate the MCB the phase to neutral or earth insulation would need to have an resistance of just a few ohms and to operate the RCD an resistance of a few Kohms. Both these values are easily detectable with a multimeter and a deciscon on the safety of energizing can be made. Once the system is energized the safety of the system and its insulation is down to its design, maintenance and operation.

However if you are trying to ascertain the condition of the insulation then its resistance (measured with high DC voltage) may be helpful but interpreting the results will be difficult.

At higher system voltages (above 11kV) there is evidence that high DC test voltages cause damage to plastic insulated cables.

My view is use a decent multimeter to check your insulation.

Michael Gilligan, yes, I noticed the lettering does seem odd on that megger, there was a mine version which was lower voltage (I have one) but they were black.I alsao have an older wooden one with "Royal Engineers" branded on it! Now we know there was also an aircraft, and a train model too! The needle position is random till the handle is wound, so could be anywhere on the scale. Sorry Simon Williams< I can't agree on robustnes, I have spent literally thousands on modern digital equipment, and have chucked it all out when it failed, and not replaced it, but I am still using an AVO8 and a megger from the early seventies! As to digital equipment being more accurate, don't believe it, check your specs and you will find that your test gear has a given accuracy at a fixed temperature only, because its electronics is sensitive to ambient changes, the AVOs and meggers use wirewound resistors which are unnafected by temperature, and anyway, the actual difference in claimed accuracy is so small as to be irrelevant, but the AVO will hold its accuracy over a much wider range of ambient temp. As to sensetivity, they all measure within the parameters of the selected scale. My problem with digital meters is that they are trying to measure values which are very often analogue! try watching a capacitor charge up with a digital meter, and looking for the little needle dips which indicate breakdowns at certain voltages, or going around tapping and rattling to find a loose connection or broken flex, ok, you can press hold and see a reading for a varying voltage, but on an analogue meter, you can watch it happen.

Thinking about insulation? Take a whole house approach to insulation. When contacting insulation companies for multiple quotes, homeowners often have done some research. If the research is misleading or not applicable to their structure, they can make the mistake of paying for an insulation treatment which yields little to no benefit. These 10 questions about insulation are in addition to the obvious top 10 you find on google such as:

1. How can you help me identify moisture transfer, and is there a moisture issue creating a food source for mold?  

2. How can you help me identify air transfer which is making the home drafty and making some rooms colder than others in winter and hotter in summer?

The company is the world’s best leakage current monitor supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

3. What are the existing types of ventilation, and how are they calculated?

4. What gaps in the pressure barrier ( building envelope) exist at present and what action can be taken to seal them?

5. Type of existing insulation, please identify the R-Value and Material?  

6. Pros and cons of adding new over existing insulation?  

7. Did you find or attempt to locate any entry points of critters, birds, bugs etc?

8. Did you identify and provide a solution for multiple air escape paths / routes?

9. What areas for improvement need to be completed by me the homeowner that your company does not provide paid services for?

10. Cost comparisons of different insulation materials and breakdown of costs and breakdown of sq ft treatment ?

Make sure to ask for bids to be submitted (in writing) with the following:

• The coverage area to be insulated
• The installed R-value (not the thickness but the final R-value when everything is completed)
• The total cost (is it an hourly rate, fixed price or based on coverage area?)
• The schedule
• Expressed warranties for the work
• Potential issues the contractor might face during the installation process

Ask the insulation contractors things like:

• How long have you been in business?
• Do you have a license (if necessary) to operate in the state?
• Are you a member of the Better Business Bureau?
• Do you have insurance to cover any accidents while working on the project?
• How do you handle customer complaints?
• What types of insulation do you install?
• Do you have references I can contact?

Tackling Air Leakage : Making your home cooler in summer and warmer in winter while lowering gas and electric bills.

Green Attic Insulation can insulation over the attic floor or between stud cavities of walls, but insulation cannot solve air flow. It can only prevent heat escape. Let that dominate the conversation through the remaining information.

Insulation and Air Leakage – Unintentional or Accidental Air Movement – Ask your insulator about air leakage.

Often referred to as draftiness, unintentional and accidental air movement account for a host of undesirable conditions, which can be treated and resolved. Green Attic Insulation has offered comprehensive whole house approach energy audits for many years now.

In the process our our years of case studies, we have compiled valuable information on the variables of creating a comfortable home.  Some simple and affordable fixes, where the treatment plan would be similar for most homes on the block, and complex and seemingly insurmountable problems which took thermal and blower door testing. This being said, we are careful not to place a percentage on energy bill reduction without achieving meaningful improvements to strengthen the air barrier. Air sealing will stop air infiltration.

What do you absolutely need to know by the time you finish reading this? A stable, healthy, consistent, comfortable , and conditioned living space is the goal.  Treatment of the air flow, heat flow, and moisture flow through the conditioned space are the clear objectives.  Each of these three have a considerably different treatment application; however, some treatments may overlap reducing the movement of one while directly targeting another.

It is essential to break down these three types of movement into their own conversations and separate the diagnosis and treatment of each.  This is a separate but equal conversation when discussing insulation.

Starting with Air Flow. Also referred to as the “pressure barrier” of the structure, air flow ( unintentional / unwanted air flow ) through the conditioned living space is the cause of over-running the furnace in the winter and cooling system in the summer.

Air Movement / Air Infiltration Methods Whole house approach identifying methods of Air Exchange ( per hour)

1. Wind effect  
2. Stack effect.
3. Combustion and Ventilation

Insulation and Wind Effect – Think the flag in the yard blowing in the wind. Ask your insulator about wind effect.

Wind / Breeze  creates a positive pressure on the windward face and negative pressure on the non-windward facing walls, which pulls the air out of the building. Wind causes infiltration on one side of a building and exfiltration on the other. Wind effects can vary by surrounding terrain, shrubs, and trees.

The most effective treatment would be a continuous air barrier around the entire exterior of the home. This is not achieved with more insulation between stud and drywall cavity.

We find the most vulnerable point of the structure is the seam between the foundation and siding. This can be achieved with exterior caulking or one part spray foam ( depending on the size of the gap and the aesthetics of the building material.

The untreated seams at the foundation level will result in cold first floors around the parameter of the home and cause substantially higher heating costs as cold air is being drawn in through breaks in the pressure barrier.

Wind defense is often overlooked due to seasonal restrictions. For example, the caulking around the window, doors, facia, and any other seams of the external structure cannot be done when the temps are below 40 Degrees per manufacturer recommendations for adhesion and proper curing.

So if a contractor meant to schedule the air sealing of the exterior and spring and never got around to returning, the homeowner ( while the home is new and beautiful) has a major air leakage problem, and resulting higher cooling and heating costs for many years to come.

Insulation and Stack Effect -Think Lava Lamp or Coffee Mug – Insulation ,the mug material, the lid is the ceiling.

The “stack effect” is warmer air moving upward in a structure. This happens in summer and winter, but is most pronounced in the winter because indoor-outdoor temperature differences are the greater. Warm air rises because it’s lighter than cold air. The rising warm air creates positive pressure above, neutral pressure between, and negative pressure on the lower levels.  Reduction in pressure in the base of the building, forcing cold air to infiltrate through open doors, windows, or other openings. The stack effect basically causes air infiltration on the lower portion of a building and exfiltration on the upper part.

Indoor Combustion and Ventilation – Indoor bath fans, dryers, range hoods, etc.

Mechanical ventilation air movement varies by device, and can cause substantial pressure differences. A common bathroom fan generates between 90-110 CFM ( cubit feet per minute , a cubic foot is about a basketball of air) , while a range hood over the stove can draw as much as CFM.  These devices must be balanced with air intake or result in unwanted air flow through the building envelope.

Now having identified three types of air movement / air flow, we can separate treatment of the air leakage by targeting the same corresponding air movement paths. For example, partitions, pathways, and intersections of the exterior to reduce wind movement. In the attic top plates and wall to wall connections are sealed to significantly reduce stack effect. This includes recessed lighting, attic hatch or pulldown stairs.  Openings around flues are flashed and fire-caulked to prevent stack effect though flu clearance framing.

Together we can ask better questions, raise energy consumption through heat escape awareness, improve ventilation, create healthier homes and happier families.

‍

Customer Review: