What is the meaning of "Highest RATED Current" in Protective Earth Impedance Test?

Roland chung

Trusted Information Resource
#1
Either 60601-1:1988 or 60601-1: 2005 states that a current of 25 A or 1.5 times the highest RATED current of the relevant circuit, whichever is greater, shall be used to check the earth impedance.

What is highest RATED current of the relevant circuit? Whether it means the rated current of overcurrent protective device (e.g. circuit breaker)?

For our three-phase equipment (line-to-line voltage: 415V), a circuit breaker rated at 32A used. But the rating power input is 15kVA, that said the rating current for a phase is about 21A. I am confused that which current (32A or 21A) shall be considered as the basis (multiplied by 1.5) of the earth impedance test?

Please kindly share your view. Thanks
 
Elsmar Forum Sponsor
#2
Re: What is the meaning of "Highest RATED Current" in Protective Earth Impedance Test

RATED is a defined term. So from 60601-1:2005 -

RATED
term referring to a value assigned by a manufacturer for a specified operating condition.

I take that to mean that the rated current is the maximum current in normal use, excluding surges etc.

For your case that would mean that the rated current is 21A and your test current is 31.5A.

Pads
 

Roland chung

Trusted Information Resource
#3
Re: What is the meaning of "Highest RATED Current" in Protective Earth Impedance Test

Thanks for your input. But please note that either 32A or 21A in this case is RATED value. The only difference is that 32A is the rating of circuit breaker and 21A is the rating of equipment in normal use.

According to CAN/CSA-C22.2 NO. 0.4-04 (Bonding of electrical equipment), the current used for the impedance test for equipment for permanent connection to the supply, twice the rating of the fuse that is required by Part I of the CEC for the branch circuit to which the equipment is connected, up to 250 A.

Furthermore, the rationale in IEC 60601-1: 2005 said, such a current is assumed to have sufficient amplitude to cause operation of protective devices in the electrical installation (fuses, circuit-breakers, earth leakage circuit-breakers and the like) in a reasonably short time.

From mentioned above, I would think the test current should base on the rated value of protective devices (fuse, circuit breaker and the like).
 

Peter Selvey

Staff member
Super Moderator
#4
Re: What is the meaning of "Highest RATED Current" in Protective Earth Impedance Test

This is actually a rare question as in most cases the 25A test applies (i.e. most equipment has <15A breakers), and it seems Roland you have exposed a weak point in the standard.

It makes sense to use the circuit breaker (32A) as the "rated" value (i.e. test at 48A). Otherwise, you could end up with a situation where the test current is less than the rated current of the circuit breaker, which makes no sense.
 

Roland chung

Trusted Information Resource
#5
Re: What is the meaning of "Highest RATED Current" in Protective Earth Impedance Test

Thanks, peter. Regarding the circuit-breaker, I have another question. That is how to determine the overload test current when circuit-breaker is used as the protective device of transformer.

According to the 60601-1:1988, transformers having OVER-CURRENT RELEASES as protective devices are loaded so that the test current in the circuit is as high as possible according to the trip current stated by the manufacturer of the OVER-CURRENT RELEASES but does not cause the releases to operate, the test being continued until a steady thermal condition is obtained.

When I reviewed the datasheet of circuit-breaker which certified to IEC 60934, I found that there is also an operating characteristic (break-time/ current) for circuit-breaker, just like the pre-arcing time/ current characteristic of fuse.

Therefore, I would think that whether the overload test current is equal to maximum current which the circuit-breaker is just capable of passing for, say 1 h based on the specification can be accepted? It seems no difference between circuit-breaker and fuse since both of them are current-type devices.
 
Last edited:
#6
Re: What is the meaning of "Highest RATED Current" in Protective Earth Impedance Test

I thought that this might cause a problem.

With a rated current (of the device) of 21A and the breaker of 32A that means 50% extra current can flow, even before you take into account the time/current release characteristics of the breaker.

The ideal thing to do would be to use a lower value breaker - you will have much less energy to absorb in the transformer tests.
 

Peter Selvey

Staff member
Super Moderator
#7
Re: What is the meaning of "Highest RATED Current" in Protective Earth Impedance Test

The normal overload test is based on a simplification: rather than testing for 1000's of possible fault conditions, let's just overload to the point of the protection, since that's the worst case.

However, in high power systems the nature of any overload is likely to be limited to a few conditions. Also there is often secondary protection to limit the amount of power in any particular circuit. So, you might consider skipping the general overload test in place of specific fault tests from secondary circuit analysis.

Another aspect is that as power increases, the acceptable losses reduces. A 100VA transformer might run at 10% loss and not really be a big worry; do the same at 15kVA and you have a serious amount of wasted energy. The result is that higher power systems are built to be more efficient, which in turn means they are not as susceptible to overload as lower power systems. You can check this by using I-squared-R based power predictions at the expected worst case overload using the circuit breaker specifications.

Of potential interest is that that as current increases there can be considerable mechanical forces under short circuit conditions due to the magnetic field. It's 25 years ago that I was involved in that kind of fault testing so the levels at which that becomes significant is I'm not sure of (the test station itself in Australia had a peak current of 220,000A!). Not something that IEC 60601-1 really worries about ...
 

Roland chung

Trusted Information Resource
#8
Re: What is the meaning of "Highest RATED Current" in Protective Earth Impedance Test

Good said. Based on your one point, if a transformer just supplies a SELV lamp, can I think that the overload test is not necessary? Because the secondary side is very simple (only lamp) and no faults can lead to overload other than the short circuit of transformer.
 

Peter Selvey

Staff member
Super Moderator
#9
Re: What is the meaning of "Highest RATED Current" in Protective Earth Impedance Test

You might need to employ the "equivalent safety" argument, from memory there is no specific clause that allows you to exclude the overload test. But it does make sense.

In reality, the probability of a real overload that exactly fits the test in the standard is virtually 0 (impossible). But, when you have thousands of components it would take a huge effort to find the real worst case overload. So, it's easier to focus on the protection limit rather than the faults (this is a common situation in safety). But, some rare cases are so simple that focusing on the fault rather than the protection is more cost effective.
 
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