These are typical MCB trip curve for type B vs C from http://finolex.com/wp-content/uploads/2018...B-Catalogue.pdf

This defines the zones where the tripping occurs, ie, there is one for Thermal (slow) and Surge (fast). The X axis is the current input, and 1x, 2x, 3x, etc of the MCB rating. So a 10A MCB at 1x = 10A, 2x = 20A and so on.

As you can see, surge tripping begins at 3x-5x for B curve, and 5x-10x for C curve.

Imagine that your lighting circuit has a B10 (B curve, 10A) MCB. The light shorts out and a current of 20A flows. Thermal tripping will happen, between the time of about 15 to 210 seconds.

If your MCB were a C10, then the thermal trip will happen between 10 and 190 seconds. From the graphs, you can see that the thermal tripping is not significantly different between B and C curve MCBs.

Where there is significantly different is between where the curves start. 3-5 for B curve, and 5 - 10 for C curve. Let's take a 4x short circuit current.

B curve trips between 0.015s to 30s, C curve between 1.5s - 15s

So how do you get past being unable to get B curve MCBs? The answer is you get a lower rated C curve MCB.
Taking the same example above, where the lighting shorts, and 4x (40A) current flows. B10 still trips between 0.015s - 30s, but if you use a C6 MCB (40/6=6.67 aka 40A is 6.67 times the rating of C6) your tripping time drops tremendously.


A B20 MCB at 3x overload (60A) becomes 3.75x if you use a C16 MCB. Not the same, mind you, but more similar to each other. But a B10 at 3x overload, becomes a 5x when using a C6 which brings the exact same timings.



This is actually why the wires you use are extremely important. Wires must be able to withstand the heating of a short circuit, until the MCB thermally trips. As you can see from the top of the Y axis, the value is in the hour range at low overload ~1.1x factor. Always see the Current Carrying Capacity of wires at 70°C (as wires heat up when carrying current) according to the proper standards. And as wires heat up, their CCC decreases. Edit to add : this is why wires are specified big, 4 mm² for water heaters. The wires may be hot, or embedded in concrete with no cooling, or many wires run together causing more heat in the wires. The wire in the water heater will have a more or less known temperature, so 2.5 mm² can be used by the manufacturer for the maximum current at the highest temperature they will reach.

In summary, B curve and C curve are only significantly different at the Surge, or Inrush current when starting inductive devices.

Hope this furthers the discussion. Any mistakes are my own. Please let me know so I may make corrections.

This post has been edited by stormer.lyn: Jan 24 2022, 11:07 PM

WH = Water heater (instant type), SH = Storage heater (water tank type)
Suruhanjaya Tenaga specifies ALL types of water heaters to be wired with a minimum of 4 mm² cable. In your instance, there is no possibility for 2.5 mm² to become 4 mm², become 2.5 mm², if you are conforming to ST rules. In other words, the wiring from the DB right into the WH/SH must be 4 mm² minimum all the way, no matter if there is an RCCB in between, or RCBO, or switch, or whatever. If you don't conform to this, you don't conform to ST rules.

If you think you can use a 13A socket, or don't want to use the Centon connector (I don't, I make direct connections), or use 2.5 mm² cable, then you can but it is entirely your responsibility. People here are posting about what you should do following the rules, but you seem to want justification on why you shouldn't. Sorry, can't do that. Wires internal to the WH/SH does not need to conform to ST wiring rules, and ceo684's post repeated below explains why.

I've said "4 mm² minimum" twice. Wire selection actually has a criteria : At full current draw of the device 1. The wire can only reach a maximum of 70°C, and 2. The voltage drop across the length of the wire must be less than 10% of the supply voltage. If either fail, you need to increase the wire size. Dagnabbit, ninja'd by ceo684

Yes, this is fine. The hose must not be made of a conductive material end-to-end to reduce the risk of electrocution.
In theory, using a plastic fitting at the outlet is also permissible as the insulator, should you be using a metal hose (They do this with storage water heaters) But I would not recommend this, as I want the longest insulator possible for safety.

Just curious... is this your opinion or are there actual rules? Your previous post was "....required by ST if you are not mistaken". I get where you are coming from when you say the metallic nut can be touched, but I checked my instant heater and the hot outlet is already made out of plastic. So, to me the metallic nut is okay. But I would welcome any actual text that says otherwise as I like following the rules. Bad things happen when rules are not followed. Bad things happen when rules are followed too, but at least no one can say stormer.lyn is the dumba$$ who couldn't follow simple rules.



Apologies for using a layman's term to describe the isolation barrier unit.

The isolation barrier for storage heaters is because typically, the hot water pipes are copper/metal until the tap fitting, which is also metal. That would be direct contact to electricity in case of a fault in the WH. Water in itself, without soap, is not that conductive. As you say, the isolation barrier mimics a 1 m non-conductive path, which is enough to prevent most electrocutions.

I agree with you. Always safer is better than not. Thanks for your insight.

cwk0713 Better not to use the hose with the metal nuts you have. Even if it is a 0.000001% chance, go for a safer solution like an all plastic connecting hose.

The developer has to terminate it and not leave it hanging loose, so a socket is installed to be utilised for general use.
If you want to use that socket for a water heater, it is your responsibility to make it fit for purpose by using the proper connector.
Similar to a socket in the kitchen that you may want to use for a high wattage oven - your responsibility to make the socket change.

An analogy is your car can be used to transport goods, but it is up to you to use a commercial vehicle to comply with the laws.

Please don't use that excuse that the socket was already there; it is very unsafe to use a 13A/15A socket for water heaters.