continuity, insulation and isolation testing

[stuart sutherland]

Hi all,
First post in here so hello to all.

I am doing some home learning and want to know more about the difference between continuity, isolation and insulation resistance testing.I understand continuity testing is basically measuring between two points in a circuit and establishing current flow. So should this always give a low resistance if circuit is good?

Problem i am having is between isolation and insulation. I cant find a good article explaining the difference between them,

Thought best to get a good reply from the guys that know.

Thanks again.

Stuart

 

[Harald Kapp]

From this discussion:

Perhaps the difference in English is that when we insulate something we put some material or abstract substance around it to separate it from other things; whereas to isolate it we simply move it away from other things.

With respect to electricity both mean that the flow of current is inhibited and I've seen these two words being used rather indiscriminately.

So should this always give a low resistance if circuit is good?

This is not necessaryily the case. Resistors are used to limit current in an electrical circuit and depending on the circuit even a high resistance can still mean a good circuit. Continuity typically means a rather low resistance. What exactly is deemed "continuous" again depends on the voltages and currents involved in normal operation. As a rule of thumb: the higher the rated current, the lower the resistance (to keep losses low since the dissipated power is calculated from P=I²*R).

 

[Martaine2005]

Hi Stuart,
Continuity is simply a circuit that is connected, current will flow from one point to another. Think of it as checking a fuse or light bulb. If that fuse is blown, there is no connection from either side of the fuse and therefore no continuity.

Isolation is quite simply as the word suggests. A circuit is isolated, therefore no current can flow. Think of isolating the mains supply before you change a light switch or alter the circuit. An isolated circuit is therefore safe to work on.
Isolated circuits from each other is another area of safety.

Insulation is another word that is self explanatory. Insulated wire has plastic or silicone coating. Un-insulated wire will be bare copper for example.
You can touch and move a household extension lead because it has a covering (insulation). If it was bare wire you would get an electric shock or the cable would short circuit. Causing the consumer unit to open a circuit and stop continuity, and is now isolated.
Hope that explains a very brief indication of all three.
It does get quite complicated when testing for various reasons but that is for another day.

Martin

 

[Martaine2005]

As pm'd to me.
"
Thank you for your kind reply.

I assumed that's what insulation testing was. If everything is insulated ok and we apply a voltage for the connections in this test am I right in saying that the resistance will be large?

Does this also apply for isolation. Circuit isolated, low current, therefore high resistance?

Hope that makes sense.

Thanks again

Hi stuart,
Insulation testing is done in meggerΩ. So yes, large.
Isolation, if a circuit is isolated (turned off) then NO current can flow.

Resistance has many factors.
Basically a short wire will have low resistance and a longer wire will have a higher resistance.
But it also depends on the thickness too.

Sorry to have copied your pm here but it's the place to ask questions.

There's a plethora of information on the 'net' about this subject and is way to large to answer in great detail.

Martin

 

[Harald Kapp]

Isolation, if a circuit is isolated (turned off) then NO current can flow.

Isolation, as used in this context, is a special case of insulation, the insulating material being e.g. air, vacuum, the base material of a PCB etc. Any kind of insu-/isolation is only good up to a certain voltage, the breakdown voltage. Above that voltage the insulation medium will break down and current will flow, e.g. in the form of a spark or flash. This is why insulation/isolation testing is done with a high voltage (to ensure protection at lower voltages) but not unlimited high (to avoid breakdown).
In isolation testing one typically does not expect infinite resistance aka zero current flow. It is standard to allow a certain maximum current (very small, in or below the mA range) to flow to account for small leackage currents that are almost unavoidable. An isolation tester will ramp up the test voltage and monitor the current. As long as the current is below the limit it will continue to ramp up the voltage until the set max. voltage is reached. It will then hold the voltage for a defined period of time and ramp down again. Should the current exceed the limit, the isolation tester will immediately shut down and give an alert. The limits for max. voltage and max. leackage current are defined on a case-by-case basis with respect to applicable safety standards e.g. EN61010.

 

[Harald Kapp]

Sorry to have copied your pm here but it's the place to ask questions.

I think that's perfectly fine.