power loss @ socket but MCB still on

[DPG]

I don't get why they are only classed as MF if you de-rate them a bit. Surely they are either rated at thate specified current or not. Are they saying that they are only rated at full current if they are available for checking regularly to make sure they haven't burnt out?? Daz

 

[LineProducts]

I don't get why they are only classed as MF if you de-rate them a bit. Surely they are either rated at thate specified current or not. Are they saying that they are only rated at full current if they are available for checking regularly to make sure they haven't burnt out?? Daz

Can of worms here guys!
In simple terms there are two British Standards that have different requirements: BS 60998 specifies Requirements for connectors. BS 5733 specifies requirements for Maintenance Free products. The requirements for BS5733 regarding heat build up/voltage drop are a lot stricter (include testing at twice rating for long periods of time).

This extra requirement makes sense when you consider that Mf relates to inaccessible locations so you want 0 chance (or as close) of failure.

MF was only introduced to BS 7671 with 17 amendment one.Much of the confusion is due to a certain manufacturer promoting their products as MF when they actually meant that you can not carry out maintenance (tighten screws) rather than the new MF which means higher spec for longer life to be used in inaccessible locations.

In answer to your question, it's not that they are expected to burn out at rated current but an effort to minimize any risk that they may.

This is much like an engineer specifying a beam to survive a 1 in 100 year event which in certain circumstances is fine or higher spec to survive a 1 in 1000 yr event.