Testing PIT

[mstrlucky74]

Not sure if any of you are familiar with this( see pic) but two questions.
1. What exactly is done when they do primary injection testing?
2. Would this apply to a 225A MCB in a panel? Says field adjustable breakers so not sure if the MCB is adjustable. Thinking it has adjustable trip setting so it would apply to the MCB's.

Thanks

 

[infinity]

This is from Megger who makes such test equipment.

Primary current injection testing is utilized in high current/high voltage scenarios found at large electrical installations such as substations. A large current (between 100A and 20,000A depending on system specifications and test requirements) is injected directly on the primary side of the electrical system such as a circuit breaker. The objective of the test is to identify how the system operates under various levels of current load.

Primary injection testing is suitable for testing over-current trip relays attached to a circuit breaker. By injecting the current into the system we can measure if the breaker will trip or fail, and how long the current is live before the circuit is broken.
Circuit breakers can go long periods of time without activation, and failure at the moment of activation can cause catastrophic damage to the electrical system. Circuit breaker testing with primary current injection on a breaker that has not triggered in some time is the best way to recreate the real operating conditions of a current spike.

 

[ATSman]

Not sure if any of you are familiar with this( see pic) but two questions.
1. What exactly is done when they do primary injection testing?
2. Would this apply to a 225A MCB in a panel? Says field adjustable breakers so not sure if the MCB is adjustable. Thinking it has adjustable trip setting so it would apply to the MCB's.

Thanks

#1 is answered in previous post.
#2 it depends on the breaker vintage.Most older MCB with magnetic trip units have adjustable dials (1 thru 10) on each phase for the instantaneous trip function only. The LTD function is normally fixed. On the newer style SS breakers the trip units got more sophisticated and the LTD, STD, GF and instantaneous function were adjustable by little dials on the trip unit. They also added a little socket to plug in a secondary injection test set. This mostly applies to MCB 100A and above.

 

[Jraef]

Most of what we call MCCBs (Molded Case Circuit Breakers) sold for simple load centers and panelboards are Thermal Magnetic “fixed trip” breakers, meaning the trip settings are set and tested by the factory and NOT field adjustable. So that statement would not apply in probably 99% of applications where you would see a 225A Main Circuit Breaker on a panel.*

When you get into “industrial” and larger commercial systems where coordination might be more critical (and your budget can afford them), you can get MCCBs with Electronic Trip Units (ETUs) that allow you to set the trip values in the field. Traditionally those were only available in 400A frames and above, but more recently the concept has crept down into 100A frames too, plus the cost difference has dropped to where they are more affordable in larger frames. The danger in having adjustable trips is in someone responding to a tripped breaker by increasing the settings. The theory is that in an industrial / large commercial setting “under engineering supervision”, where there are established policies against doing such a thing, that’s less likely to happen. In theory...

The spec you are reading is likely an indirect way for the engineer to discourage their use, because having to have every one of them certified via primary injection testing after installation will make it very unlikely that the cost can be justified without someone explaining the risk/benefit situation to those paying the bill.

* Side note: The term “MCB” has multiple meanings now. I’m assuming by context that you meant Main Circuit Breaker, but it also stands for “Miniature Circuit Breaker” in the rest of the world and now has crept into our lexicon too. So we should all think about how we use that acronym now by making sure we include clear context.

 

[Ingenieur]

as mentioned most mcb are not adjustable

in simplest terms you apply a v across the cb, this results in i limited only by the cb and conductor Z, which are very low
the v is variable, usually a variac feeding a step down xfmr of high turns ratio

assume cb + test wiring ~ 10e-6 ohm
variac 120 in 0-10 out
xfmr 100:1
variac out 1 v
xfmr out 0.01 v
i = 10e-3 / 10e-6 = 1000 A
power is low ~ 10 va, the heft is the xfmr sec and conductors

they usually have taps for various i outputs
they also time opening and record trip current so you can verify curve compliance
you usually set the desired level and push a button

to do pit on a 200 A cb at 1300% is 2600 A
a large portable unit on wheels

200 A range output are like suitcases
we use it to do sec inj testing since most of our work uses ct's
if we need 600 wrap it 3 times

 

[zog]

Not sure if any of you are familiar with this( see pic) but two questions.
1. What exactly is done when they do primary injection testing?
2. Would this apply to a 225A MCB in a panel? Says field adjustable breakers so not sure if the MCB is adjustable. Thinking it has adjustable trip setting so it would apply to the MCB's.

Thanks

It means you need to inject current and test the trip timing of all functions. Typically Long time is tested at 300% of rated current so your 225A breaker would be tested at 675A and the trip time compared to the manufactures time current curves to see if it is still within band. If the breaker has other functions like ST, INST, GF, etc...those functions must also be tested.