HRC Prefered Over MCCB

HRC Fuse as well as MCCBs have their pros & cons.

HRC fuses are:

Fast acting for a heavy short circuit (typical operating time < 5 ms)

Low Capital Cost

Current Limiting

Low Let-thro energy

But....

Recurring (replacement) cost is more

Increased downtime (due to replacement)

Only Short circuit protection

Possibility of single phasing

Not possible to check the functioning of the fuse non-destructively

MCCBs are:

Moderately fast acting (typical opearting time <10ms)

Low operating cost

Lower downtime

Can have overload, short circuit, earth fault, under voltage protections

Will not cause single phasing

Can be tested at site for proper functioning

But...

Capital cost is more

Non-repairable (use & throw device)

And...

I do not think of any reason why HRC fuses are preferref over MCCBs for capacitors. I have commissioned many Capacitor Feeders with MCCB back-up

So also, there is no logic in the PCC incomer being HRC fuses and the outgoings MCCBs. Even for discrimination, one needs an ACB in the incomer.

Many Indian consultants are driven by age old practices which may not be correct.

Thanks for the reply...

But Panel I was mentioning was a sub switchboard fed from the PCC

Is there any advantage if we provide HRC fuse as incomer..?

They showed me a clause from the diary issued by the Chief Electrical Inspectorate

"When Switchfuse is available at the incomer side outlet switch rating shall not be less than 1/10th of the incomer fuse rating.Maximum rating of fuse outlets shall be 1/2 of the incomer fuse rating."

"When MCCB is provided at the incoming side the outlet switch rating shall not be less than 1/5th of the incomer setting".

I have also referred to these "guidelines" specified in Chief Electrical Inspectorate Association Diaries. I have written many times in the past to all these associations to authentiate their writings. But, nobody replied. Pl. ask the concerned publishers of the diary as to from which National/International Standard that they got these stipulations. I am sure they wouldn't answer.

The technical reasoning for such sizing of fuses or MCCBs could be due to discrimination requirements. Otherwise, the upstream fuse might blow for a downstream fault. But, that definitely does not advocate the use of only HRC fuses or only MCCBs in a given location. It is after all a customer's choice.

Remember, MCCBs are developed as a replacement for HRC fuse units. As such wherever HRC Fuses were used, MCCBs can convedniently replace them, provided other requirements such as I2t withtstand of downstream devices are properly taken care of.

Ask <...They...>.

One more clause that made me confused is that

"When the switch board is connected to 1600 kVA transformer the minimum outlet switch rating shall be 630 A

If it is connected to 1250 kVA transformer 400 Amps switches can be permitted."

I am not able to understand what is the reason...?

Pl. ask the publishers of these so called diaries. I am of the strong opinion that these so called 'guidelines' only misguide people.

I support the above recommendations. In my opinion this has basis, both commercially and technically.

In a Main MV panel of say of 1600 Amps rating, if tap off outgoing feeders are rated like say 63 Amps / 100 Amps, a) the outgoing MCCB KA ratings need to be sized higher than that provided in down stream sub switch boards, & b) the frame size of panel will be bigger for higher current rating and taking small power tap off will increase cost of main MV panel.

More importantly, at Main MV panel level, fault current levels are much higher. 63 Amps feeders connected here will be provided small dimensioned Copper or Aluminium cables which will not have required with stand capacity for uncleared faults downstream of the breaker. This can lead to a potential fire hazard.

Hence I am strong advocate in judicious sizing of feeders at main MV panel and at subpanel levels.

I am thankful for the explanation provided.Do you have any ideas about my other queries..?

"Incomer of the Power Switch Board (PSB) which is fed from PCC is protected using HRC fuses while the outgoing feeders of PSB are protected using MCCBs."

The PCC have MCCB outgoings.The PCC outgoing MCCB and the incomer fuse of PSB have same rating."

"When Switch fuse is available at the incomer side outlet switch rating shall not be less than 1/10th of the incomer fuse rating.Maximum rating of fuse outlets shall be 1/2 of the incomer fuse rating."

"When MCCB is provided at the incoming side the outlet switch rating shall not be less than 1/5th of the incomer setting".

"Incomer of the Power Switch Board (PSB) which is fed from PCC is protected using HRC fuses while the outgoing feeders of PSB are protected using MCCBs."

Sufficient information is not available to peer review the design basis. Please Check whether in original design concept, Switch Isolator was proposed but instead got converted to Switch disconnector fuse (SDF) during execution. Under standard conditions, MCCB should be preferred to SDF because of various advantages already enunciated by EE65 and others.

The PCC have MCCB outgoings. The PCC outgoing MCCB and the incomer fuse of PSB have same rating."

Maintaining same rating of breakers / Switches at both ends of a feeder is a standard and good engineering practise.

"When Switch fuse is available at the incomer side outlet switch rating shall not be less than 1/10th of the incomer fuse rating. Maximum rating of fuse outlets shall be 1/2 of the incomer fuse rating."

"When MCCB is provided at the incoming side the outlet switch rating shall not be less than 1/5th of the incomer setting".

Already clarified in my earlier reply. Please consider these as guidelines and not as rules to be strictly followed. Each and every installation will have its own requirements and the designer will have to apply his experience and choose the correct design basis.

In Indian textual context, switch means either SDF or MCCB.

A disadvantage to fuses on 3 phase systems is the risk of single phasing. In a capacitor bank, all of the capacitors are in essence single phase connections anyway, so there is no real need to open all 3 phases together in the event of a short circuit, which is the only kind of fault you will see in an APFC system. So the faster clearing speed of fuses can MAYBE buy you some reduction in collateral damage should one of your caps decide to fail catastrophically. In practicality, probably not, because the amount of time involved in a capacitor failure is extremely low, so the chances are whatever caused one to fail will cause many to fail anyway.

As to mains on a switchboard, fuses often offer a coordinated approach to high Available Fault Current situations. Often times fuses are added TO a main MCCB as current limiters.

It's not a good idea to second guess someone, no matter how old you believe their thinking is, if you are not the person who is signing your name to the overall project. They may have or had specific knowledge of the installation that you are not aware of.

thank you for the reply.

My major problem with MCCB's is and always will be "ease of resetting". With capacitors it's all to easy to over stress switches when they operate.

OK I'm an old git, dragged up in the quarries. I'm stuck in my ways!

A 1000KVA transformer would have 100KVAR PFC. Most plants were 24/7 so static units were used. 100KVAR = 133A we used 630A fuse switches fitted with 200A fuses as standard.

Thank you for replying....

<...Does any body knows what might be the reason....?...>

The simple route to an answer is to ask <...the consultants ...>.

Electrical inspectorate of various states have differences in their rules .My own experience was the Kerala Inspectorate was the toughest and more technically correct.HRC fuses are fast acting and let through current is much lower as compared to MCCB but to day fast acting MCCB "s are available but rules are not revised.