a=3;
b=2;
c=4;
Tpec=1200;
λ=Fit[{{500,0.11},{700,0.15}},{1,T},T]/.T->650

S=0.5*(2*(a*b+a*c+b*c)+2*((a+2d)*(b+2d)+(a+2d)*(c+2d)+(b+2d)*(c+2d)));
Svnejsi=2*((a+2d)*(b+2d)+(a+2d)*(c+2d)+(b+2d)*(c+2d));

Rth=(d/(λ*S));
Tokoli=20;
σ=5.67*(10^(-8));
ϵ=0.9;
α=5;
rce={((Tpec-Tplast)/Rth)==S*(α*(Tplast-Tokoli)+ϵ*σ*((((Tplast+273))^4)-(((Tokoli+273))^4)))};
volba={Tplast->65};
Solve[rce/.volba][[-1]]
dosd={d->0.3};
Tplastvys=Tplast/.Solve[{((Tpec-Tplast)/Rth)==S*(α*(Tplast-Tokoli)+ϵ*σ*((((Tplast+273))^4)-(((Tokoli+273))^4)))}/.dosd][[-1]]

Pust=((Tpec-Tplast)/Rth)/.dosd/.Tplast->Tplastvys;

ro=130;
cp=1000;
rce=ro*cp*D[T[x,t],t]==λ*D[T[x,t],{x,2}];
poc=T[x,0]==Tokoli;
okrVlevo=-λ*D[T[x,t],x]==(1/(S/.dosd))*3*Pust*(1-(E^(-t)))*0.5(Tanh[-0.2(T[x,t]-1200)]+1)/.{x->0};
okrVpravo=-λ*D[T[x,t],x]==α*(T[x,t]-Tokoli)+ϵ*σ*((((T[x,t]+273))^4)-(((Tokoli+273))^4))/.x->d/.dosd;
tmax=24*3600;
res=NDSolve[{rce,poc,okrVlevo,okrVpravo},T[x,t],{x,0,d/.dosd},{t,0,tmax}][[1]]
Plot3D[T[x,t]/.res,{x,0,d/.dosd},{t,0,tmax}]

Plot[0.5(Tanh[-0.2(T-1200)]+1),{T,1100,1300},PlotStyle->Thick]

Plot[UnitStep[-(T-1200)],{T,1100,1300},PlotStyle->Thick]

Plot[1-(E^(-t)),{t,0,10},PlotRange->All];