MCCB FEEDER AND EARTH FAULT LOOP IMPEDANCE TEST 1

[PRincez]

We just installed a new UPS suit fed from a Siemens 3VL MCCB and the switchboard is 300m distance from the load (UPS). The MCCB is fitted with an Electronic Trip Unit (Siemens ETU22) set at 1x In.

My question is; as we are now commissioning the system, should an Earth Fault Loop Impedance (EFLI) test be conducted from the MCCB fed load given the 300m distance to source? that has earth fault protection device? Is it common practice is conduct EFLI test on MCCB fed loads?

When we I looked at the regs (BS7671) the Zs values given are for MCB's not MCCB. The MCCB data sheet does not indicate what trip curve it's classed as i.e. 'Type B, C or D'. I could possibly request an loop impedance value table from the Vendor (if available) but don't want to labour it if not required.

Injecting current into a live plant earth grid seems an iffy thing to do to me.

Comments will be appreciated.

 

[Kiribanda]

PRincez,
In my opinion the requirement depends on your system earthing. Also whether it is mcb or MCCB it is the "protective device". BS7671 is for electrical installations in buildings therefore always mcbs are used and not MCCBs.You have to measure both EFLI & PFC at the load end.
1) If your earthing system is TT, then you have to ensure tha EFLI x Ia=50V where Ia is the current required for effctive operation of the protective device with in 1000s so that max safe touch voltage 50V is not exceeded. That is why we use RCCDs for TT systems.
2) If your system is TN, then you have to ensure that EFLI x Ia < U0 where U0 is the nominal voltage between phase & earth and Ia is again the current required for eff operation of the MCCB within 5s.
But in either case you should get the trip curve or the TCC of the subject bkr to check the operation time.

 

[ScottyUK]

BS7671 is for electrical installations in buildings therefore always mcbs are used and not MCCBs.

Not true. BS 7671 applies to low voltage installations. It's certainly geared toward domestic / commercial / light industrial but it applies to the bigger LV installations too.

It's not unusual to perform an ELI test on an MCCB feeder. You'll need to look at the OEM's breaker curves to determine the limiting values of ELI, or use something like Amtech's ProDesign software (buggy as the Scottish islands in June, and the tele-sales people latch on to you like blood-sucking leeches). Don't forget that the latest BS 7671 requires that the ELI is calculated at 90% of nominal system voltage, not 100%.

 

[davidbeach]

Also, I believe the MCCB has different definitions in different parts of the world.

 

[ScottyUK]

I've always thought that was something we agreed on!

Ours are typically rated from 100A or so, up to about 2000A; have little or no capability to hold in against a through fault; and are (officially) non-maintainable. I've seen the Schneider NSX range for sale wearing a Square D badge in North America.

 

[davidbeach]

Well, based on the "always mcbs and not MCCBs" statement, I was taking that usage of MCCB to be something other than the molded case circuit breaker I'm familiar with. Typically starting at 15 or 20A here and running up to where they sort of bleed into the ICCB (insulated case) in the 1000A + range. Certainly not maintainable. I know there are "miniature" circuit breakers of some sort over there but wouldn't expect them to be used for building installations.

 

[ScottyUK]

Our MCBs (as distinct from MCCBs) are the type of device found in a panelboard for final distribution, typically from a few amps up to about 63A and in 1, 2, 3, or 4 pole varieties. I don't know many US ranges by name but the Square D QO series and the old Federal Stab-Lok spring to mind. We don't really see the term ICCB over here.

 

[Skogsgurra]

I assure you that large parts of Europe - and most of the non-British part - cannot distinguish between MCB and MCCB.

I did a quick search and here are the results:

"MCBs are miniature circuit breakers with optimum protection facilities of overcurrent only.
MCCBs are Moulded case Circuit breakers,with protection facilities of overcurrent, earth fault.

MCB is a shortcut for Miniature Circuit Breaker, usually mounted as withdrawable type and used for small electronic equipment loads.
MCCB, is a Molded Case Circuit Breaker,usually used for motor and transformer loads, The latter is bolted mounted and usually has higher interrupting current rating than MCB's

MCB miniature circuit breakers generally used for control circuits (Approximately 0.5 Amps - 15amps) power supply application.
MCCB Molded case Circuit breaker - power application Generally 3 pole or 4 pole Start from 15Amps to 400Amps( some time more )

MCB = miniature circuit breaker upto 100 ampere.
MCCB = molded case circuit breaker upto 1000 ampere.

MCB means moulded circuit breaker.
MCCB means motor control circuit breaker.

MOULDED CASE CIRCUIT BREAKER
Load Line range of Moulded Case Circuit Breakers are designed and manufactured to world-class standards. Loadline series MCCBs provide overload and short-circuit protection for all applications. The thermal & magnetic elements, adjustable over a wide band, make these MCCBs ideal for any distribution application.

MASTER CIRCUIT BREAKER

MCB (Miniature Circuit Breaker)
MCCB (Moulded Case Circuit Breaker)

MCB- Miniatuer circuit breaker - low breaking capacity generally for lighting load.
MCCB- Moulded case circuit breaker- generally for short circuit protection in power DB
MPCB- Motor protection circuit breakers for motor protection application with different class

MCB In NEMA world, this used to exclusively mean "Main Circuit Breaker", meaning the primary circuit breaker in a group or panelboard / switchboard. In IEC world, it means "Miniature Circuit Breaker", typically a DIN rail mounted circuit breaker in a small format that is limited to 100A, and is intended for use in installations with low fault current capacity (under 25kA) unless protected by upstream fuses.
MCCB Was originally an IEC term meaning Molded Case Circuit Breaker, but has been very well adopted in NEMA world now, at least at the engineering level. Many rank and file electricians still are not familiar with that acronym because its widespread use in NEMA world is fairly recent.
MCP A NEMA world acronym for Motor Circuit Protector, specifically describing a magnetic-only MCCB (no thermal trip elements) that is to be used STRICTLY as part of a factory assembled, listed and tested motor starter assembly. They are often mistakenly specified and used in user-built assemblies, but this is not acceptable in the NEC (US) and CEC (Canada). In IEC world, a Motor Circuit Protector can sometimes mean a completely different device, although generally the MCP acronym is not used to avoid confusion.

MCB : it is protected against only due to short circuit fault. not for any other Overload fault.
MCCB : it is applicable for short-circuit , O/L protection, Also it has E/F protection feature.
MPCB : Is is specially made for motor protection application, i.e. Overload & Shortcuit protection"

I cannot say that this makes me any wiser. There is a certain consensus as to what one means when using acronyms like MCB, MCCB, MPB, MCC etcetera.
But there's enough spread and uncertainty in usage that I would (and don't) not use those TLAs and FLAs when I want to make sure that there isn't any misunderstanding.











Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[krisys]

So far for the MCCB feeders supplying to an equipment like UPS, I have not seen the people doing the earth fault loop impedence (EFLI Test) test. However, proper earthing continuity between the upstream source and the downstream equipment shall be ensured.

 

[ScottyUK]

I'd test the ELI of the feeders to the UPS. Any ELI readings from the synthesised output of a static UPS feeding a downstream MCCB would be questionable at best. I might have a bit more faith in the ELI from an old-school rotary UPS, but I haven't seen one of those for a few years.