Enough Fuse Discrimination?

[KPG]

Evening all,

I am a newbie to this site and also to electrical work. I am currently a young road lighting technician at the local council and am studying for a HNC in electrical engineering, a G39 and a Diploma in road lighting standards. I have been in contact with a consultant and he has brought up a question that my little-to-no practical knowledge of electrical installations.

Currently we have a distribution pillar that feeds adjacent Lighting column with a 10A BS88 fuse, and is then sub-fused in the control column with a 6A BS88. The power consumption is 60W from the lamp. The cable size going into the column is 2.5mm PVC cable. TN-C-S supply.

I have spoke to my tutor and he recommends that if the 6A Fuse would have enough discrimination if if was a fast blow fuse. But I cannot find anything on the fuses that indicate what type it is.

If anyone could help me or point me in the right direction, that would be appreciated.
e.g. What formulae should I use to be working this out?

Also if there is any more information needed to make this more clear. I will be happy to assist.

 

[Sparks-R-Us]

Don't know if this will help...
J-type fuselinks are based on the British Standard BS88 Part 5. They have been designed specifically to satisfy the requirements of the electricity supply industry for feeder pillars and other distribution applications. They have compact dimensions and employ wedge tightening contacts.
They are fast-acting industrial type current-limiting and energy-limiting fuselinks and comply with the time-current gates of IEC 269 at the faster end of the characteristic. Low fusing factors make them ideally suited to the protection of cables and feeder circuits. They are generally more compact than the alternative NH-type fuselinks, have lower watts loss and have a gU type characteristic. NH-type fuselinks are based on the German standard DIN 43620/1, and have gG and gL characteristics.

 

[IQ Electrical]

There are 2 types of BS88 curves, a gG type and a slower gM type.

Have a read of this: http://www.tlc-direct.co.uk/Book/3.8.6.htm

 

[Gordon]

you haven't enough information to answer this question, you need to know the Zs or earth fault loop impedance and calculate the prospective fault current. PFC = U/Zs. When you know the PFC you can look in the big red book fig3.3A & fig 3.3B to get the operation time for the protective devices. Fig 3.3A shows a 6A BS88 fuse will operate within 0.1s on a fault current of 35A, fig 3.3B shows a 10A fuse requires 60A to operate within 0.1s so if the fault current is less than 60A you will achieve discrimination. if it is higher you can't.

 

[Chr!s]

Theres two discrimination factors overload and short circuit, most fuses you will find work to the rule of thumb ratio 1.6:1. using this factor total discrimination will normally be achieved. if you want to be more specific you would need to the pfc and the i2t for the protective devices.